To study the process of laser clad Al/Ti composite powder above 7050 aluminum, a three dimensional model was established, which can simulate the molten pool size, the temperature gradient, cooling rate and the shape control factor in the condition of different defocusing amount. The simulated results show that the width and depth of the molten pools are positively correlated to the defocusing amount, but negatively correlated after the defocusing amount reaches 20 mm. Both the max temperature gradient numerical value and the best cooling condition appear in the Z direction, which also indicate the growth direction of grains mainly concentrates on the Z direction. The cooling speed reaches the highest and the grain size is small when the defocusing amount is 40 mm. The cooling speed is lowest and grain size is big when the defocusing amount is 80 mm. Moreover, the shape control factor is biggest and the crystal microstructure is columnar when the defocusing amount is 60 mm, and the shape control factor is smallest with cellular crystal microstructure when the defocusing amount 80 mm. Finally, the simulated results about both the grain size and the crystal microstructure of the molten pool are verified by the experiment study.