According to the test requirements of the inner wall of a pipeline, an optical system of pipeline endoscope is designed, which is based on particular optical feature of gradient index lens and microscope. The system has the advantage of large depth of field. In the condition of required magnification, the depth of field of the system is increased by 200 than the initial system, which is achieved by the radial gradient in the reflective index of gradient index lens. The system is more suitable for pipeline internal surface inspection based on the feature of the nearly spherical surface of image plane. Combined with CCD, the digitalization test of pipeline endoscope is realized. Clear image of the defects in the inner wall of a pipeline, such as the crack and abrasions, can be checked conveniently by monitor in a larger extant of depth of field.