Abstract: Steam hammer pressure is solved though the simplified calculation.A mathematical model of main steam system in a million-kilowatt nuclear power plant is established using PIPENET software,and the model is from steam generator to main steam header.Steam hammer occurrence and reduction is simulated through transient calculating function.Parameters are calculated including maximum steam hammer pressure,maximum steam hammer load,the time of occurrence of the maximum steam hammer load and the position of occurrence of the maximum steam hammer load.The effect of straight pipe length and valve closing time on steam hammer is analyzed.The conclusion is that the shorter straight pipe is or the longer valve closing time is,the lower energy of steam hammer is.