The influence of bottom heat source on natural convective heat transfer characteristics in a two-dimensional square cavity fully filled with a homogeneous porous medium is numerically studied by the lattice Boltzmann method. In this physical model, the upper wall of porous cavity is set to be a cold heat source, and the bottom wall is designed as a local hot heat source. Both the left wall and the right wall are set to be adibatic. Specifically, the effects of both the position and size of bottom heat source on the properties of natural convective heat transfer are analyzed. The numerical results show that the position and size of bottom heat source have great influences on the characteristics of natural convective heat transfer, and there also exist the best position (a=4/16) and optimal size (b=0.75) of the bottom heat source for the maximal convective heat transfer intensity (Numax 10.35) and heat exchange capacity (Qmax 5.69).