Abstract: The T welded joint is widely used in engineering, and the size of weld leg directly affects the quality of the products. In order to explore the effects of weld leg size on the joint strength, the experiment was made with the specimens of different sampling position and different sizes of Q345 T welded joint based on tensile test, and the mechanical properties of T welded joint were tested. According to the equal-intensity theory, the optimal weld leg size was calculated at the different load. The comparison between the theoretical calculations and the experimental results showed that the measured value basically agrees well with theoretical value.