In the absence of real-time data of the stations after the earthquake，this paper attempts to make some constraints from the source in order to improve the accuracy of ShakeMap． The causative fault of the Yushu earthquake is approximately vertical ( dip 83°) ，and the source process in time and space was distinctly dispersive． On the basis of the line source model of surface rupture and the magnitude decomposition method of main shock， we gnerate the ShakeMaps which takes the site effect into consideration． We compare the ShakeMaps of the above methods with intensity of investigation． The result shows that magnitude decomposition method of main shock based on source process，considering Shakemap of each sub － event，is feasible for disaster emergency response，decision-making and rapid disaster assessment after the earthquake．