Wavefront coding (WFC) technology can extend the depth of focus of an optical system, which makes the application of this technology to space cameras extremely attractive. This paper introduces the application of wavefront coding to three-mirror anastigmatic optical systems. A transition model is established, in which the secondary mirror is regarded as the wavefront coding element and redesigned according to mathematical calculation. A comparison of modulation transfer function (MTF) behavior between the traditional system and the innovative system with wavefront coding technology is provided. The MTF behavior of both on-axis and off-axis field-of-view points remains the same in spite of the extended depth of focus. It is also observed that the system becomes very insensitive to aberration related to defocus through WFC technology. Errors in optical design and its improvement are discussed. The linear transition model is proven to be an acceptable one. Finally, an extension factor for the depth of focus is defined, and its effect is presented graphically.