Result of the focal spot measurement on the high power laser with single lens imaging method is reported in this paper. Focal spot of the high power laser is sampled by an ellipsoid concave mirror, and after passing through a single lens, it is imaged on the CCD system. With testing data to be processed, reliable focal spot shape and intensity distribution are obtained. In this way, real time test with the focal spot of the high power laser is realized. The testing result has been validated in other way.