At present, seismic strengthening design reference period of the existing building is usually equal to 50 years in China, sometime this is uneconomic and unreasonable. In this paper, determining principle of seismic strengthening design reference period for the existing building with different importance is presented. The seismic strengthening design level of the existing building is put forward. After the shape factor of intensity probability distribution function is used to represent the seismic hazard characteristic of different areas, the seismic hazard curve formula of design acceleration Amax and earthquake influence coefficient αmax are deduced according to the seismic hazard curve of intensity. The seismic strengthening design ground motion parameter for the existing building with different importance is researched in detail by use of hazard curve formula of seismic ground motion parameter based on seismic hazard characteristic zone. At last, the method and the calculation step are explained by a calculation example. The result shows that for the existing building with different design reference period, using same design parameter is unreasonable in different seismic hazard characteristic zone, and the method is more scientific than the code method.