Leaf area index (LAI) is one of important parameters of quantitative description of leaf growth situation. Compared with effective LAI which is influenced by gathering condition among leaves greatly, actual LAI can reflect the real crop growth status more accurately. Terrestrial laser scanning (TLS) can obtain high precision crop structure information quickly such as height and biomass. But in terms of leaf area information, the major part is effective LAI. Reference to the idea of voxelization, the method was put forward for estimating actual leaf area of maize according to the leaf and its projective plane’s mathematical relationship in voxel, which makes full use of TLS’s advantage of gathering vertical information. The point data which can perform maize actual growth state as the data source directly. And the maize point cloud was divided into a series of leaf unit by voxel. The leaf unit area based on the leaf and its projective plane’s mathematical relationship in voxel were calculated. Then the maize actual leaf area was calculated. The different TLS was used to obtain four sample’s maize point clouds which were different in varieties and scales in both Beijing City and Hebei Province, respectively. The point obtained from sample points (No. 1, 2, 3) is individual maize point, and the actual LAI was calculated in different voxel sizes. The main conclusions are as follow: calculated LAI has a good correlation with measured LAI as all of decision coefficients are greater than 0.8, it proved that this method is credibly. The optimal voxel size is 0.17cm, 0.15cm and 0.15cm, respectively. Relevant with the optimal voxel size, RMSE is 61898cm2,44058cm2 and 42844cm2 respectively, the relative error between the plant total leaf area is -2.678%, 0.619% and -0.474% accordingly. The result means that this method has a high accuracy. In the sample point No.4, the relative error between calculated LAI and measured LAI is -14.663%, the calculated LAI is smaller than measured LAI. All in all, the method about estimating actual leaf area of maize according to the leaf and its projective plane’s mathematical relationship in voxel is feasible with high precision.