The present invention discloses a CMOS device of reducing charge sharing effect and a fabrication method thereof. The present invention has an additional isolation for trapping carriers disposed right below an isolation region. the material of the additional isolation region is porous silicon. Since porous silicon is a functional material of spongy structure by electrochemistry anodic oxidizing monocrystalline silicon wafer, there are a large number of microvoids and dangling bonds on the surface layer of the porous silicon. These defects may form defect states in a center of forbidden band of the porous silicon, the defect states may trap carriers so as to cause an increased resistance. And with an increase of density of corrosion current, porosity increases, and defects in the porous silicon increase. The present invention can reduce the charge sharing effect due to heavy ions by using a feature that the defect states in the porous silicon trap carriers, the formation of a shallow trench isolation (STI) region and a isolation region underneath only needs one time photolithography, and the process is simple, so that radioresistance performance of an integrated circuit may be greatly increased.