Electrostatically actuated circular diaphragm exhibits an instability known as pull-in phenomena, short circuit caused by pull-in with contact is destructive to electrostatic micropump operation. In order to avoid pull-in instability caused by high driving voltage, pull-in voltage should be calculated accurately. Therefore a reduced order model (ROM) based on numerical calculation and mode superposition method was constructed, which could take account of residual stress and geometric nonlinearity. The pull-in voltage and pull-in position computed by ROM have been compared with the existed results and the results calculated by 3-D finite element analysis. It showed that the ROM sped up computational process without sacrificing the accuracy. The effects of different parameters such as residual stress, initial gap, diaphragm radius, diaphragm thickness on pull-in voltage and pull-in position were also investigated by the ROM. In addition, the design criteria of pump diaphragm were obtained, which provided an effective theoretical basis for the optimization design of electrostatic micropump.