A kind of reversal shear interferometer has been designed to measure the temporal change of spatial coherence of KrF excimer laser with an unstable resonator. By this method, we can get the spatial coherence degree from a single measurement. At different shear distance, using slit and photodetector. The stepwise increase of the spatial coherence degree at intervals equal to the transit time of the resonator was observed. Moreover, the ray matrix has been used to analyze the propagation of partial coherent light and the coherence widths of successive passes are calculated.