The stress intensification factor is a key parameter in the piping stress analysis of 45° lateral tee in the vent system. In order to make the calculation more accurate, six kinds of stress intensification factors of welded 45° lateral tee commonly used in low-pressure vent system were calculated by using finite element analysis software NozzlePRO. The results were compared with those calculated by formulas of 90°unreinforced fabricated tee in ASME B31.3 Process Piping and ASME B31J Standard Test Method for Determining Stress Intensification Factors (i-factors) for Metallic Piping Components. The results show that the stress intensification factor calculation formulas for branch pipe and main pipe in and out of the plane of the 90°unreinforced fabricated tee in ASME B31J are still applicable to the welded 45°lateral tee. When the torque at the tee cannot be ignored, the torsional stress intensification factor of the branch pipe and main pipe of the fabricated 45° lateral tee should be calculated using the finite element software. This research results can be used asareference for piping stress analysis.