In this paper, the simulation model of ablation and phase change of SUS304 stainless steel targets irradiated by continuous laser or continuous/modulated composite laser is established. The temperature rise curve and phase change distribution of the steel target irradiated by the composite laser or the continuous laser are calculated and verified by experiments. The ablation process of steel target is described by the dynamic grid method, and the curves of the ablation and phase change depths versus time are obtained. The simulation results are compared with the experimental results, and the results of temperature rise, phase transition distribution and ablation are in well agreement. According to the simulation and experimental results, the perforation of the steel target under the above laser irradiation can be divided into two ways: vaporization perforation and melting perforation according to the power density. When the output power is the same, the speed under the 2.55 kW/0.85 kW composite laser is 41% faster than that under the 3.4 kW continuous laser for the 20 mm steel target. The mass transfer mode is the joint action of melting and vaporization stamping, and the molten material splashing phenomenon is obvious in the ablation process, which forms a certain degree of positive feedback to the temperature rise and the phase change and ablation process of the steel target.