This paper proposed a semi-analytical approach for analyzing the problems of the tunnel water inflow. The approach was developed by using the classical ground water theory. The groundwater problem in a finite domain is transformed to a problem in an infinite domain by the image method and the superposition principle. Using the convolution-deconvolution method, the time-variant inflow problem is transformed to a constant flow problem. The calculation formula was developed for the conceptualized hydro-geological models for the tunnel inflow problems. The computation is simple and fast in comparison with the other 2D and 3D numerical methods. The parameters required in the approach are few and easily estimated. Two cases of the tunnel water inflow were simulated by the approach. The calculated inflows agree well with the general trends of the measured inflow data.