In this paper, the particle-in-cell/Monte Carlo method simulation algorithm with independent intellectual property is described in-depth, and discharge properties of the JAERI 10 A multicusp ion source are investigated by using the algorithm. The effects of discharge parameters (such as air pressure, discharge electrode position, magnitute of the filter magnetic field, discharge voltage, etc) on volume production efficiency are analyzed. The results show that in order to obtain high volume production of negative hydrogen ions, filter magnetic field should not be too large, and the discharge voltage should not be too small, but the filter magnetic field and discharge voltage should be appropriately selected, and primarily regulating air pressure and discharge location simultaneously can optimize the ion source.