Abstract: The peak shape of time-of-flight mass spectrum contains information of ion initial spatial and velocity distributions. Information how ion initial spatial and velocity distributions influence the peak shape will help to analyze ion formation mechanism. A peak shape simulation program of TOF-MS based on probability was developed, and the effects of ionic initial spatial and velocity distributions on ionic peak shape were studied. The ionic spatial distributions can be acquired from ion peak shape under deviated space focus condition. The program provides a feasible approach for ion source analysis.