2D simulation of the main influence factors on the charge collection characteristics of Silicon On Insulator(SOI) PIN microdosimeter was performed with TCAD software. The transient current in the microdosimeter induced by 3 MeV alpha particle was calculated at different applied voltages(from 10 V to 50 V), doping concentrations and alpha incident directions. The simulation results show that the transient current increases with the increase of reverse bias voltage due to the decrease of the carrier recombination effect; and the space charges induced by alpha particle are almost collected in 1 ns with 10 V applied to the n+ region at the charge collection efficiency nearly 100%; and the transient current decrease when the doping concentration of each region increases.