The purpose of this study was to explain the phenomenon of seawater intrusion (SWI) by numerical simulation and analysis. In this paper, a numerical method was introduced for solving the onedimensional advection-dispersion equation by the explicit finite difference method, and the corresponding convergence conditions and the boundary attenuation factor, , were given, which can be convenient in studying the impact of different influencing factors on seawater intrusion. Additionally, the effects of six hydraulic factors on seawater intrusion were explored under steady state, namely, hydraulic conductivity, longitudinal dispersivity, hydraulic gradient, flux, evaporation and rainfall. Moreover, two different seawater intrusion types were also studied, namely active seawater intrusion and passive seawater intrusion. Meanwhile, the control variates method was adopted to explore the influence of a single factor on seawater invasion. According to the calculated results, with the increase of hydraulic conductivity, longitudinal dispersion, flux and evaporation, the degree of it will be aggravated, but the rainfall can play a role in mitigating seawater intrusion. Besides, under active seawater intrusion conditions, the growth of the numerical value of hydraulic gradient will aggravate seawater intrusion, while under passive seawater intrusion conditions, it will slow down the degree of invasion. All in all, the results and achievements are consistent with the actual situation, which can explain the phenomenon of seawater intrusion.