In order to analyze the tightness of bolted flanged connection system accurately,the method of equivalent pressure,equivalent axial force and deformation compatibility equation were used to study the gasket compression stress of bolted flanged connections under the external bending moment.The analysis results indicate that when the method of equivalent internal pressure is used to calculate the gasket compression stress,the external bending moment is equivalent to the internal pressure and the internal pressure of the system is increased,but the calculation result can not reflect the uneven distribution of gasket stress in the bolted flange connection system under external bending moment.When the method of equivalent axial force is used to calculate the gasket compression stress,the calculation result can reflect the uneven distribution of gasket stress along the circumferential direction of the pipe under external bending moment,and the compression stress difference is increased between the tension side and compression side of the gasket.Among them,the gasket stress is decreased on tension side and increased on compression side,so the gasket stress on tension side must be considered closely when studying the tightness of the bolted flanged connection