In the optical design process of off-axis three-mirror system of large aperture and long focal length, the problem of light blocking appears. The traditional optimizing methods can reduce the efficiency of optical design, and have negative impact on the image quality. Practice shows that the problem can be solved with ZPL macro instruction. The design indexes of reflective collimator include the aperture of 500 mm, focal length of 4000 mm, wavelength range of 400～1400 nm, specified field angles of 2°×1°, and the spot diagrams is better than 6 μm. According to the design indexes, the initial optical structure parameters of coaxial three-mirror anastigmat(TMA) are computed. The final design scheme is obtained with the help of ZPL macros of Zemax software. Results show that the spot diagram is better than 6 μm and wavefront aberration of every specified field is better than λ/25, which meet the design indexes. ZPL micro instruction assist design can control the structure of off-axis multi reflective mirrors system, and it can avoid the light blocking, improve image quality.