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As shown in fig, if I use the keyval, then my MWE does not work well.

What's wrong with my MWE?

documentclassarticle

usepackagekeyval
usepackageexpl3
usepackagexparse

makeatletter
defBUPTthesiscntitlepage#1setkeyscntitle#1
define@keycntitletitlelongdefBUPT@value@title#1
makeatother

makeatletterExplSyntaxOn
seq_new:N l_@@_title_seq
NewDocumentCommand settitle m
 seq_set_split:Nnn l_@@_title_seq \ #1 

dim_new:N l_@@_title_width_dim
NewDocumentCommand printtitle 

 group_begin:
 @@_title_name_format:
 tl_use:N c_@@_title_name_tl
 group_end:
 group_begin:
 @@_title_format:
 @@_get_max_width:NN l_@@_title_width_dim l_@@_title_seq
 dim_add:Nn l_@@_title_width_dim 10p@ 
 dim_log:N l_@@_title_width_dim
 parbox [t] l_@@_title_width_dim 
 
 seq_map_inline:Nn l_@@_title_seq
 @@_underline:Nn l_@@_title_width_dim ##1 \ 
 
 group_end:


cs_new_protected:Npn @@_get_max_width:NN #1#2

 group_begin:
 dim_zero:N #1
 bool_until_do:nn seq_if_empty_p:N #2 
 
 seq_pop:NN #2 l_tmpa_tl
 exp_args:NNV @@_get_width:Nn l_tmpa_dim l_tmpa_tl
 dim_gset:Nn #1 dim_max:nn dim_use:N #1 l_tmpa_dim 
 
 group_end:

cs_new_protected:Npn @@_get_width:Nn #1#2

 hbox_set:Nn l_tmpa_box #2
 dim_set:Nn #1 box_wd:N l_tmpa_box 


cs_new_protected:Npn @@_underline:Nn #1#2
 underline hbox_to_wd:nn dim_use:N #1 hss #2 hss 

tl_const:Nn c_@@_title_name_tl Title: 
cs_new:Npn @@_title_name_format: Large bfseries 
cs_new:Npn @@_title_format: Large bfseries 

ExplSyntaxOffmakeatother

BUPTthesiscntitlepage%
 title = AAAA \ BBBLaTeX B

begindocument
settitleAAAA \ BBBLaTeX BB
printtitle

makeatletter
longdefBUPTtitletempBUPT@value@title
makeatother

settitleBUPTtitletemp
printtitle
enddocument

When you set the title like settitleBUPTtitletemp, the BUPTtitletemp macro is not expanded. So the splitting at \ fails. Expansion only happens when you actually typeset the lines in an hbox (where you can't have a linebreak).

An easy fix in this case is to make sure the content that is passed to settitle is at least expanded that far that an unexpandable token occurs at its start:

exp_args_generate:n Nnf 
NewDocumentCommand settitle m
 exp_args:NNnf seq_set_split:Nnn l_@@_title_seq \ #1 

The result is as expected

First and foremost: @@ is an internal convention used in .dtx files to denote a common prefix in functions and variables and should not be used in production code.

The simplest ways to cope with your problem is to avoid keyval and use the more powerful features in expl3.

You can so store the title as a sequence, splitting it at \, instead of having to do it later.

Also a tabular is easier than parbox, as it doesn't require to measure the items.

documentclassarticle

usepackageexpl3
usepackagexparse

ExplSyntaxOn

keys_define:nn danger/BUPT 
 
 title .code:n = seq_set_split:Nnn l_danger_title_seq \ #1 ,
 
seq_new:N l_danger_title_seq

NewDocumentCommandBUPTthesiscntitlepagem
 
 keys_set:nn danger/BUPT #1 
 


NewDocumentCommand printtitle 
 
 group_begin:
 danger_title_name_format:
 tl_use:N c_danger_title_name_tl
 c_space_tl
 group_end:
 group_begin:
 danger_title_format:
 begintabular[t]@ l @
 seq_map_function:NN l_danger_title_seq __danger_underline_endline:n 
 endtabular
 group_end:
 

cs_new_protected:Npn __danger_underline_endline:n #1
 
 underline #1 \
 

tl_const:Nn c_danger_title_name_tl Title: 
cs_new_protected:Npn danger_title_name_format: Large bfseries 
cs_new_protected:Npn danger_title_format: Large bfseries 

ExplSyntaxOff

BUPTthesiscntitlepage
 title = AAAA \ BBBLaTeX B,


begindocument

printtitle

enddocument

Note that settitle is defined as

seq_set_split:Nnn l_@@_title_seq \ #1

which meanse its argument should be a set of tokens given in braces. So

settitleAAAA \ BBBLaTeX BB

will take AAAA \ BBBLaTeX BB as the argument, then split it at \ and push AAAA, BBBLaTeX BB into l_@@_title_seq.

However, when using

settitleBUPTtitletemp

the argument is just BUPTtitletemp (NOT its value), which has no \ explicitly. Therefore, the whole title AAAA \ BBBLaTeX B will be pushed into l_@@_title_seq as a single item. This will lead to errors since the following function such as @@_get_max_width:NN does not allow \ in its argument.