Using wedge-splitting method, the changes of moisture content and double K fracture mechanical properties of three strength grades (C20, C30 and C40) concrete immersed in free water environment for 0 h, 2 h, 5 h, 24 h and 120 h were studied, so as to provide support for the safety evaluation of concrete structure in water environment. The results show that the higher the concrete strength grade is, the slower the water absorption rate is, the lower the moisture content is, and the longer soaking time it takes to reach saturation state. The initial cracking fracture toughness and unstable fracture toughness of concrete decrease linearly with the increase of moisture content. The initial cracking fracture toughness of C20, C30 and C40 concrete in saturated state is 29.6%, 23.2% and 33.4% lower than that in dry state, and the unstable fracture toughness of C20, C30 and C40 concrete in saturated state is 22.7%, 23.9% and 33.8% lower than that in dry state, respectively. Under the same moisture content, the initial cracking fracture toughness and unstable fracture toughness of C20, C30 and C40 concrete decrease similar basically. The P-CMOD curves obviously have a linear elastic non-crack initial stage, a stable expansion stage and an unstable expansion stage. With the increase of moisture content, the slope of linear elastic stage and stable expansion stage gradually decreases, and the curve of unstable expansion stage drops more and more gently. The critical crack opening displacement decreases gradually with the increase of moisture content, the deformation capacity and toughness of concrete decrease gradually.