A new method of frequency spectrum analysis using Power System Blockset(PSB) is presented.Based on the PSBFFTSCOPE function supplied by MATLAB,the input variable is reformed. This method can overcome the defect that the function can not be used to analyse the frequency spectrum of such signals sampled at variable step.And then the spectrum analysis range has been broaden in the PSB.The simulation results are accurate and indicate that the new method is feasible.