Abstract：In-situ bioremediation technology is one of the most important methods for volatile chlorinated hydrocarbons（VCHs）remediation in groundwater. Accurate simulation of the remediation process is critical for high-efficiency remediation. Nevertheless， most of the previous studies did not consider the natural attenuation of VCHs， which would lead to the inaccuracy in prediction. Therefore， this paper has proposed a Monod-Natural attenuation（M-Na）model of in-situ bioremediation considering the natural attenuation of VCHs to overcome the limitation of current VCHs in-situ bioremediation model. The source code of the M-Na model was coupled in RT3D in the form of user-defined reaction module. In this paper， hydrogen was provided by injecting lactic acid for in-situ bioremediation of tetrachloroethylene（PCE）contamination in aquifer to verify the accuracy of M-Na model and analyze the influence of controlling parameters（including lactic acid injection concentration and rate）of the remediation process. The simulation results have shown that the concentration of ethylene（ETH） increased about 10% due to the natural attenuation of VCHs， which could not be neglected. Meanwhile， the lactic acid optimal concentration is 30 mg·L-1 and the optimal injection rate is 200 m3·d-1 for the study area. This study is expected to provide accurate simulation model for in-situ bioremediation of groundwater contaminated by VCHs.