There are many problems in the construction of ocean islands and reefs, including the shortage of materials, high cost of transportation and materials. Therefore, a new type of seawater sea sand volcanic slag concrete (SSAC) was developed to promote the development of marine construction. Through the notched three-point bending beam test, the influences of different initial notch-to-depth ratio (0.2, 0.3 and 0.4) and concrete mix component types on the fracture mechanical properties of SSAC were analyzed. The variations of fracture parameters, such as double K fracture toughness, fracture energy and softening curve were also obtained. The results indicate that the load-crack mouth opening displacement (P-CMOD) curve of concrete with volcanic slag coarse aggregate is similar to that of ordinary concrete, but the descending section of volcanic slag concrete P-CMOD curve becomes steep due to the effects of coarse aggregates. The fracture toughness of SSAC is smaller than that of ordinary concrete, and it is not affected by the initial notch-to-depth ratio. Under the influence of seawater and sea sand, the fracture toughness and fracture energy of SSAC are 14.30% and 6.77% higher than that of fresh water river sand volcanic slag concrete. Based on experimental data, the analytical expressions of double K fracture toughness and softening considering the influence of concrete mix component are established. The calculation results are in line with the test ones.