Abstract: The peak profiles of ions from ionization of CH₃I clusters in the nanosecond laser field were simulated using a peak shape simulation program of time-of-flight mass spectrometer (TOF-MS). Ionic initial position and velocity distributions, which depended on the formation mechanism, were obtained by comparing the experimental and simulated peak shape, and the ion source was also illustrated. C²⁺, I²⁺and I^3+-ions come from Coulomb explosion, while C⁺, CH_x⁺(x=1,2,3), I⁺ and CH₃I⁺ ions were formed by multiphoton ionization (MPI). H⁺ is due to both Coulomb explosion and MPI, and a small portion of I⁺ origin from electron impact ionization. In addition, the acceptance ratio of ions was studied by the simulation method.