Cassegrain system is the most classic reflective optical system in telescope systems. The imaging point of a regular Cassegrain system is outside the system and thus can be tested using an interferometer. However, the imaging point of the improved Cassegrain system lies between two mirrors, so that it cannot be directly measured. It can be tested in two steps based on the system characteristics: an accurate test is made for the primary mirror; the image quality is evaluated based on the system′s dispersion spot. Such a system is measured by a profilometer and tested by Zygo interferometer and the optical shadow method. A special structure is used to test accurately. The surface figure should be modified based on the measured results in the test process to meet the requirements on the optical system′s imaging quality.