In view of the continuous phase plate that was designed by GS algorithm with the characteristics of complex surface and difficulty in processing, a method for evaluating the phase plate was put forward by using both the surface shape and the focal spot parameters. Based on the actual design results, we calculated Peak Value (PV) and Root Mean Square (RMS) value of the phase plate and the characteristics of its focal spot. The feasibility of using peak value and RMS value as evaluation was analyzed. Finally, compared with the focal spot corresponding to three different phase plates, the PV, RMS value and power spectral density of focal spot were analyzed.