documentclass{article}

usepackage{keyval}
usepackage{expl3}
usepackage{xparse}

makeatletter
defBUPTthesiscntitlepage#1{setkeys{cntitle}{#1}}
define@key{cntitle}{title}{longdefBUPT@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}}

begin{document}
settitle{AAAA \ BBBLaTeX BB}
printtitle

makeatletter
longdefBUPTtitletemp{BUPT@value@title}
makeatother

settitle{BUPTtitletemp}
printtitle
end{document}