Abstract: The multiphoton ionization of CH₃I molecule with 532 nm nanosecond laser was studied by a homemade laser ionization time of flight mass spectrometer. The influence of laser intensity on the full width at half maximum W_1/2 of I⁺ ion peak and the peak position of CH_q⁺ (q=0-3) ions were investigated in details. When laser intensity was increased from 1.4×10¹⁰W/cm² to 5.2×10¹⁰W/cm², the W_1/2 of I⁺ ion was broaden from 113 ns to 426 ns, while the peak position of CH₃⁺ was shifted forward about 52 ns. Through formula derivation and the linear fitting between the peak broadening, peak shift and intensity of I⁺ ion, the W_1/2 of I⁺ ion is proportional to I_I^+0.5, while the peak shifts of CH_q⁺ (q=0-3) were proportional to I_I⁺. At the laser focus, the Coulomb repulsion among the ions increases with the increasing of ion intensity, and the Coulomb potential due to the space charge effect has an influence on the ion kinetic energy, which results in the change of the time of ion reaching the detector. The peak broadening and shift of ions can be well explained by space-charge effect.