A porous medium with different porosity is modelled based on the random four parameters growth method and the Sparse Matrix Storage Formats is used to store the internal fluid node, the fluid solid boundary node and the physical boundary node. The simulation of lid driven flow and natural convection in a square cavity are studied and the correctness of the Lattice Boltzmann model is verified. Based on the lattice Boltzmann method, the velocity and temperature field distribution in the complex porous medium are calculated, and the relationship between velocity or temperature and each of iteration time and porosity are analyzed in detail respectively. The results show that the distributions of fluid velocity and temperature are affected by the porosity of the porous medium. With an increase of porosity, the average velocity increases under the same pressure gradient; with an increase of pressure gradient, the average velocity increases under the same porosity. At the same temperature, with the increase of porosity, the time convection heat transfer reaches a steady-state in the porous medium is gradually reduced. The maximum temperature appears at the high temperature surface, and the bigger the porosity near the high temperature wall, the greater the volume of the high temperature part. With a decrease of porosity, the average temperature of the porous medium decreases under the same condition.