The tree crown volume is difficult to measure and calculate because of its irregular shape. A calculating method of tree crown volume, named voxel simulation method, was presented based on 3-D laser scanning point clouds data. The main idea of this method was to use a certain size of voxel to estimate tree crown. Firstly, tree crown was cut into segments with a k step-length along tree height. For each segments, all the points were projected to a plane that perpendicular to tree height. Subsequently, the plane was divided into pixels with size of k×k. The effectiveness of each pixel depend on whether there were points projected in it. Then the effective pixels were counted and recorded as T. Finally, formula T×k×k×k was used to calculate the tree crown volume. When the k was equal to a tenth of crown diameter, the calculated crown volume was tending to stability. This algorithm did not need to consider the crown shapes of different tree species, which could reduce human’s subjective factor influence. It could be used to calculate tree crown volume based on 3-D laser scanning technology. 