A new phenomenon is observed when we use YAG 355 nm laser to ionize water/methanol mixture molecular beam and detect the ions with time-of-flight mass spectrometer. The peaks position of mass ions shift when changing the laser delay time to the molecular beam or singly changing laser energy while keeping other conditions not changed. It can be determined by analyzing mass spectra that the peak's shift does not mean new ions appear.The reason is deduced to the voltage fluctuation on the extract electrode of TOF when ions passing through the grating electrode and parts of them are being absorbed. The ion's shift altitude is proportion with the ion's intensity,and the ion's total flight time is shortened with ion's intensity increasing. The numerical simulation of charged-particle's movement in electrical field also supports the deduction and is consistent with the experimental results.