In order to resolve the fracture problem of reactor pressure vessels (RPVs) under extreme high temperature, the creep fracture behavior of a RPV with a semielliptical surface crack has been studied in which the constraint effect was considered. The constraint effect near the crack tip was represented using the C*-A2 two-parameter method. The constraint-based creep crack propagation law was developed, and the creep crack propagation process of the cracked RPV was simulated at 900 K using the finite element method. The results show that a high constraint level occurred near the crack tip under extreme high temperature, which led to loss of fracture toughness of the cracked RPVs. The constraint effect level near the crack tip could be improved, and the creep transition time could be shortened because of the growth of the creep crack. It is illustrated that creep behavior and constraint effect are significant factors endangering the structural integrity of cracked RPVs by markedly accelerating crack propagation and shortening the lifetime of cracked RPVs.