Because of the differences in the mechanical properties of the welded joint district, the deformation law and forming characteristics of aluminum alloy Tailor Welded Blanks (TWB) joint is quite different compared to traditional sheet metal forming. This kind of mechanical properties mismatch often leads to the forming property of aluminum alloy TWB lower than the base metal, which greatly limits the popularization and application of aluminum alloy TWB in automobile manufacturing industry. Based on the 2024-O aluminum alloy TWB of Friction Stir Welding, through the tests combined with the finite element method, study the influence law and influence mechanism of mechanical properties mismatch on the formability of aluminum alloy TWB. Microstructure and hardness test of aluminum alloy FSW joint were carried out. According to the distribution of the hardness of welded joint, the FSW joint can be divided into Thermal Mechanical Affect Zone (TMAZ), Heat Affect Zone (HAZ) and Base Metal (BM). Finite element simulation of uniaxial tensile test was carried out base on the finite element model of FSW joint. Using the finite element model, the influence of mechanical properties mismatch of WNZ, TMAZ, and HAZ on tensile property of joint is studied. The results show that the stress is lowest on minimum yield stress area, and corresponding increased on larger yield stress region. The stress will mutate if there is mismatch ratio difference. In the views of the strain energy, the results are contributed by the nonuniform property of deformation and the constraint in the FSW joint, furthermore which leads to the variation of yield strength, location of yield, tensile strength and elongation with the diversity of mechanical properties mismatch.