The fracture properties of 20g steel used as one of boiler materials, after undergoing the heat treatment of short period high temperature and thermal shock, are investigated by evaluating their fracture toughness using the crash opening displacement (COD) and three-point bending test experiment. Meanwhile, the orthogonal test consisting of five temperature stages and four different durations of thermal shock is used to explore the relationships between fracture toughness and temperature or duration. The experimental conclusions show that thermal shock plays a more significant role on affecting the fracture properties, compared with the factor of temperature. Reducing the duration, especially at the high temperature, is useful to inhibit the decrease of fracture toughness. The ranger analysis illustrates that the lower temperature does not have the beneficial effect on the fracture behaviors. Furthermore, the experimental results facilitate providing the valuable reference on the effective safely management of utilize boiler.