diff --git a/tex/pkuthss.cls b/tex/pkuthss.cls
index b839256..c7b86b2 100644
--- a/tex/pkuthss.cls
+++ b/tex/pkuthss.cls
@@ -172,6 +172,10 @@
 \fi
 
 \AtBeginDocument{
+	% Set up spacing for displayed formulae.
+	\setlength{\abovedisplayskip}{\belowdisplayshortskip}
+	\setlength{\belowdisplayskip}{\abovedisplayskip}
+
 	\ifthss@opt@pdftoc
 		% Add PDF bookmark for table of contents.
 		\let\thss@tmp@tableofcontents\tableofcontents
@@ -241,12 +245,6 @@
 	headheight = 0.45cm, headsep = 0.55cm, footskip = 1.05cm
 }
 
-% Set up spacing for displayed formulae.
-\setlength{\abovedisplayskip}{6bp plus 1.5bp minus 3.5bp}
-\setlength{\abovedisplayshortskip}{3bp plus 0.75bp minus 1.75bp}
-\setlength{\belowdisplayshortskip}{3bp plus 0.75bp minus 1.75bp}
-\setlength{\belowdisplayskip}{6bp plus 1.5bp minus 3.5bp}
-
 % Set up chapter/section/... captions.
 % The `*skip' values are not supposed to be modified by the `ugly' option:
 % the actual style of the school's guide and Word template seem to be different