Abstract: This paper is concerned with the thermal shock when the cold safety injection fluid is injected into the nuclear reactor. Based on a control volume method and k-ε turbulence model, numerical simulation was carried out to investigate the flow and the heat transfer of 3D inclined jet to cross flow at angles of 30°、60°、90°、120ånd 150° in the T-junction pipes. The pressure distribution, the temperature distribution and the flow structure were thoroughly studied. The paper analyzed the effect of geometric structure, the flow condition and the temperature variety on the flow structure, temperature distribution and thermal stress. The study found that temperature difference and velocity ratio had great effect on the thermal stress. On the upstream of the jet, thermal stress is more powerful.