A three-dimensional geometric Monte Carlo (GMC) method is proposed. By taking advantage of the geometrical relationship between the photon position and the interface, GMC can simulate the photon transportation in the whole domain rather than a voxel. Discrete voxels are unnecessary and the photon motion is calculated according to the geometrical optics. Therefore the optical transmission error induced by the voxel Monte Carlo (VMC) method can be eliminated. Also, the computation time consumed by GMC is dramatically shortened, and GMC is about 25 times faster than VMC with voxel grid size of 10 microns for the single vessel situation. Through the calculation of the energy deposition in a tissue model with the multi-coaxial vessel cluster, it is found that the dependence of energy absorption on the vessel distribution will recede when the vessel number increases at a certain blood volume fraction (BVF). The largest deviation of blood energy absorption is 4% with 20 vessels when BVF is 5%. This implies that artificial vascular distribution can be used to predict the real absorption characteristics with the same blood volume fraction instead of difficult measurement of real complex distribution of blood vessels, which is of great practical importance for the clinical treatment.