Abstract: Eleven typical seismic sections were selected from the Xihu Sag to calculate the extensional and compressional rates of the sag in different tectonic evolutionary stages in Cenozoic, by means of the balanced cross section technique. Results suggeted that the Xihu Sag be in an extension state before the 3rd and 4th members of the Eocene Pinghu Formation was deposited, and in a compression state during the deposition of the 3rd and 4th members of the Eocene Pinghu Formation to the Miocene Yuquan Formation, and had remained in an sinking state since the Miocene Liulang Formation up to present. It, in a tectonic point of view, means that the Sag was extensional before the middle Eocene, and changed into compressional since then, and the compressional strength increased successively from the Yuquan movement(T30), to the Huagang movement(T20) and to the Longjing movement(T12). Based on the depositional architecture of the depression, the tectonic evolution of the Xihu Sag could be divided into four stages, namely the syn-rifting stage from Paleocene to middle Eocene, the post-rifting stage from late Eocene to Oligocene, the inversed stage during early to middle Miocene and the sinking stage since Miocene. The tectonic evolution shows some sort of difference between the south and the north of the Sag in different stages in terms of extensional and compressional rates, that might be the reason for the differentiation of structural appearance in the south and north of the Sag.