Serial tensile tests were conducted to investigate the mechanical properties of different parts for QSTE700 square tube. It shows that the strength is improved by 10% at welded portion while the strength of flat portion remained unchanged. The strength of corner portion is 17.78% higher than that of the original steel but still lower than that of the traditional carbon steel square tube. The real fillet plays an important role in yield strength improvement. The square tube in the FE model was divided into two parts with different material properties based the test data. The simulation result indicates that the model with different material properties is much closer to the real situation.