diff --git a/ctufit-thesis.cls b/ctufit-thesis.cls
index 3769f46e802625ee4c670a10922c6b05a0bafdda..a56c5b2ef09b4810a7c1d7e7518f44d69cdd6a85 100644
--- a/ctufit-thesis.cls
+++ b/ctufit-thesis.cls
@@ -447,7 +447,6 @@
   \thm@postskip\thm@preskip
 }
 \def\@endtheorem{\endtrivlist}%\@endpefalse
-\renewcommand\qedsymbol{\textcolor{decoration}{\ensuremath{\blacktriangleleft}}}
 \renewenvironment{proof}[1][\proofname]{\par
   \pushQED{\qed}%
   \normalfont \topsep6\p@\@plus6\p@\relax