Experimental measurement was employed to study the effects of the DF high-power laser irradiation on 45#steel in different gas flow, by analyzing the surface picture and measuring the temperature histories, the effect of steel ablation was most obvious and the temperature on the rear center of the coupon was highest in the tangential air flow. The experimental results showed that melt was removed by the tangential gas flow, especially in the air flow where combustion played an important role in heating the material, and the target was cooled after laser irradiation terminated. According to the experimental results, a numerical model was established for simulating the irradiation effects of high power density laser on steel targets in different gas flow. The method of “Element birth and death” was employed to simulate the ablation effects of laser on steel targets in the air flow, where the effects of heat releasing by oxidation were included. The results of the numerical simulation agreed well with the experimental, and ulteriorly explained the effects of the gas flow in the laser irradiation.