A new method is proposed to detect the dynamic 3-d shape of object,which is based on FFT method.A sequence of deforming fringe-images can be saved on disk by CCD snap. By FFT algorithm a sequence of phase-maps can be obtained,Encoding these phase-maps in 3-D(x,y,t) field,we can get the shape of object at different time.In this paper, we present the thoretical analysis and the results of simulation caculation and experiment.