Abstract: With the sharp increase in the proven reserve of tight sandstone gas, the tight sandstone gas has become the major target in exploration and development in Xihu Sag, and even East China Sea Basin. The tight sandstone gas reservoirs are characterized by complex lithology, complex pore structure and reservoir heterogeneity, irregular mud invasion and the unclear distribution of water and gas, which made it difficult to evaluate tight sandstone reservoir correctly and quantitively. The fracturing technique is needed for testing the production ability, because the production ability is limited with conventional formation test. However many factors would affect the test resulting during the fracturing process, these uncertain factors bring difficulties in selecting the target layers. In this paper, on the basis of core analysis, combined with various logging data, DST testing data, five major factors that influence the offshore fracturing result have been proposed. In addition, the diagnostic index for fracturing the tight reservoir have been made, and a method to optimize the fracturing layer of the tight sandstone layer in the sea has been put forward. This optimization method has been widely used in the Xihu Sag and the goals to reduce the risks of fracturing has been achieved.