Bolted flange joints were simulated with the finite element software Ansys, getting the gasket stress distribution under the condition of the preload and operation, and calculating the gasket stress under different working condition of internal pressure. Due to constant fluctuations in pipeline pressure and the pipe stress complex, the working pressure, temperature, uncertain factors such as their own constraints on its tightness should be fully considered. The Monte Carlo method is used to have reliability analysis in pipeline working pressure fluctuating. Limit equation is created and according to the distribution of the variable type and limit equation using largescale software Matlab is used to repeat random sampling to calculate the probability of bolt flange leakages under different working pressures. The results show that, the additional load produced by working pressure fluctuation will have a very big effect on the bolt flange leak, that must be got seriously enough attention.