An insert coaxial thermal radiation image evaluating system includes a cage support, first lens, first cage movable frame, second cage movable frame, cage holder and light detector. The first cage movable frame is movably disposed at the cage support and connected to the first lens. The second cage movable frame is movably disposed at the cage support and connected to the light detector. The cage holder is connected to the cage support to fix the cage support to an optical substrate. The first cage movable frame is movably disposed in the cage holder. The first lens and a second lens of a metal additive manufacturing system together form a structure of conjugate foci, such that a thermal radiation generated from a high-power infrared laser irradiation zone forms according to a fixed ratio an image captured by the light detector.