This paper focuses on the performing texts in the Yuan edition Yuankan zaju sanshizhong, categorizing and analyzing Baiyueting and other six plays, and tries to find out the performing clues hidden behind many various stage directions. By doing some basic calculating works, we find the preliminary "formalization" of performance could be traced back to the Yuan times, while at the same time, there were lots of "flexible" and "individual" improvisatory stage directions, too. Based upon these two complementary characteristics, formalization and improvisation, our theory of performance of Chinese classical theatre will be enlarged and deepened.