Abstract: The shell and tube heat exchanger is widely used in engineering. The shell side is mostly complex steam-liquid two-phase flow, and it is difficult to simulate the heat exchanger with thousands of heat transfer tubes using precise grids. In order to realize numerical simulation of shell side, porous media model is commonly applied to reduce computational grids numbers. The volume porosity and surface permeability are two important parameters of porous media, and previous researchers have proposed several methods to calculate them. However, these methods either directly set volume porosity as a constant, which are not accurate enough, either are tedious even have good precision. Hence, a fast and accurate calculation algorithm named GTG (grid combined with tube geometry) was proposed in this paper. The volume porosity was calculated using this method based on the position relationships between grids and heat exchange tubes and the surface permeability was got based on the regional shortening method. It applied for rectangular and cylindrical coordinates. The porous coefficients can be updated automatically with the change of grid scheme. For verifying its correctness, porous coefficients calculated by GTG method were compared with the real values measured by CAD. The maximum error is only 4.5%, which indicates that the calculation accuracy is good. And a numerical simulation of the secondary side in steam generator was developed based on GTG method. The simulation results are in good agreement with the results of similar studies, which explains that the method is effective.