A new method developed for wavefront error (WFE) testing of an assembled space telescope is described in this Letter. The main idea of this method is to image a small bright target on the focal plane array assembly by the space telescope itself; the imaging light beams can be reflected to the focal plane array by a plane mirror in front of the aperture of the system under test and two images (one is in-focus and the other is defocused) can be obtained. The WFE of the optical system can be calculated with a phase diversity algorithm according to two images of the target on the focal plane array assembly. The residual error and limitations of this method are discussed and a simulation result is shown, which shows the application potential of this technology.