In order to study the influence of the pulse number on the topography of ablation craters. Craters were processed on the surface of the cast iron by nanosecond pulse laser. The relationship between the depth of the crater and the number of the pulses was analyzed. The crater depth almost linearly increases when the pulse number is under 10. Then the depth growth rate becomes slower and slower. When the pulse count is above 15, the quality of the crater is poor, because of the crater is deeper, and the molten metal cannot splash out of the crater. The increase of crater diameter will be slower with the pulse number. In order to get the temperature distribution and the topography of craters, the temperature field of the ablation was simulated. The simulation results are consistent with the measurement results, which demonstrate the rationality of the model.