The pre-crack concrete specimens with different relative incision depths were tested under 0, 25, 50, 75, and 100 times freeze-thaw cycles, respectively. A three-point bending test was performed on the test pieces after the freeze-thaw cycles, and the apparent changes of the test pieces were observed, and the cracks of the test pieces were observed by a scanning electron microscope and a stereo microscope. The fracture mechanical properties of concrete under freeze-thaw environment were studied from the aspects of macroscopic mechanical properties, damage deterioration law and fine microstructure. The damage variables of concrete based on micro-defects and dynamic elastic modulus were defined according to the damage theory of concrete. The results showed that the concrete freeze-thaw cycle damage is the process of initial defect development, degradation and accumulation. The concrete fracture toughness has no apparent relationship with the relative depth of the initial crack, but has a good linear correlation with the flexural strength.