In order to solve the problem of welding defects in Invar alloy welding, the technological parameters of Invar alloy lap welding (the thickness of the upper and lower plates with 1.5mm) for liquefied natural gas (LNG) ship sheet were studied. Though designing and manufacturing a gas protection box, by means of numerical simulation analysis and orthogonal experiment, the optimal process parameters of Invar alloy lap welding were obtained.The experimental results show that the porosity defects are effectively suppressed under complete shielding gas, and the optimal technological parameters for the lap welding of Invar alloy are obtained as: Laser power is 3.4kW, welding speed is 1.3m/min, defocus is +20mm, laser incident angle is 5°, and laser spot energy distribution is 2∶1, respectively. Under the optimal process parameters, the weld surface is silvery white, and there is no defect such as air holes. The hardness of the weld is smaller than that of the base metal but larger than that of the heat affected zone. The tensile strength is 417.16MPa, which is 94.8% of that of the base metal. The error between the simulation results and the test results is small, which proves the reliability of the simulation model established in this paper. This result is helpful for the construction of the later process parameter database of Invar laser lap welding.