The microstructures and the mechanical properties of friction stir lap welded Ti-6Al-4V were investigated by both experimental and numerical methods. Dislocation density evolution model was proposed to link the microstructural changes observed by scanning electronic microscope and simulated by Monte Carlo method with the mechanical properties measured by tensile tests and simulated based on dislocation evolutions. It was found that lamellar grains can be formed in the prior-beta grains in friction stir lap welding of Ti-6Al-4V. The dislocation density can be increased to 1.29 x 1014/m2-1.48 x 1014/m2 in friction stir welding, which is 1.29-1.48 times higher than the parent metal. The yield strength can reach 986.33 MPa-1055.77 MPa in as-welded state, which is 91.58%-95.66% of the parent metal. The predicted yield strength can agree well with experimental data. Higher temperature can reduce the dislocation multiplication and coarsen the microstructures in friction stir welding. Softening in stress-strain curves can be obviously found in the temperature range of 600-900 circle C.