High-power laser system has a requirement for the medium and low spatial wavefront errors of the transmitted wavefront of the spatial filter, which is a spherical lens with long focal length (> 10 m) and large aperture (> 400×400 (mm)). Two interferometric methods are analyzed and compared for testing the medium and low spatial wavefront errors of the long focal length lens with an instantaneous interferometer, using large -aperture plane mirror and concave mirror as retroflectors, respectively. The two kinds of beam path -arrangements are designed using ray tracing method. Wavefront aberration and modulation transfer function are used as merit functions. The reason for their different ability to test the wavefront is also analyzed. To evaluate the -feasibility of test methods, tolerances analyses are performed to determine the tolerances demanding the fabrication and assembly of each optical element. It is proved that a relatively short optical path with a large aperture concave mirror is satisfied for testing the medium and low spatial wavefront errors of the transmitted wavefront of the spatial filter.