We introduce a simple method primarily developed for the performance evaluation of assembled space telescope in the field. The main idea of this method is to obtain the large parallel light beam for testing with an internal point light source and a plane mirror instead of large collimator. The wavefront error of the system can be calculated with phase diversity algorithm according to two images of the internal point source. The residual error and limitations of this method are discussed. A simulation is given at last, which shows the utilization potentiality of this approach.