The dependence of various STI depths on charge sharing in 90nm dual well CMOS technology was investigated. TCAD simulation results show that increasing STI can restrain charge sharing of NMOS effectively, and 550nm is the effective depth for the prevention of charge diffusion, beyond which the collected charge almost keeps constant; for PMOS, charge sharing decreases linearly with the increment of STI depth. This conclusion is useful for irradiation-hardness.