The method suggested by Code for Seismic Design of Buildings is conservative for deep soils, while NCEER method is complexity for engineering practice, in addition, the calculation result is unsafe for deep soils.In this paper,the NCEER method based on standard blow number is simplified,and the expression of critical blow count is derived.The improved method based on the simplified NCEER method and code method is proposed,which can be expressed as a function of depth of water table and calculation point and easy for engineers to master.