Aiming at the macroscopic and mesoscopic causes of hot cracking in the 7075 aluminum alloy plasma-MIG hybrid welding process. Fishbone cracking test was applied to test the welding hot cracking sensitivity. 7075-T6 aluminum alloy sheet with 4 mm thickness was welded by Plasma-MIG hybrid welding method using ER5356 aluminum alloy wire with φ1.2 mm diameter. The brittle temperature range（BTR） of the 7075-T6 aluminum alloy was measured and the hot cracking tendency was evaluated. Combing macroscopic thermoelastic-plastic method and mesoscopic crystal plastic finite element method, the numerical simulations results show that a slight strain difference will lead to the different hot cracking sensitivity, so the appropriate adjustment of the strain difference is proposed to optimize the welding process.