Wavefront coding technology (WFC) is a kind of joint optical-digital imaging technology, which can extend the depth of focus (DOF) of optical systems and control the defocus and defocus related aberrations. Another important character of the WCT is to enlarge the error budget of optical systems. The modulation transfer function (MTF) invariance is defined in this paper and the comparison of MTF invariance among original system, wavefront coded system with original tolerance and wavefront coded system with new tolerance is presented. The new tolerance of wavefront coded off-axis three mirror anastigmatic system is presented according to certain criteria and the application of WCT is validated quantitatively.