Aiming to solve the problem of variable-rate pesticide spraying in sparse branch orchard, a method of target leaf area calculation based on mobile laser scanning (MLS) technology was proposed to provide basic data for real-time variable-rate pesticide spraying. To eliminate the influence of LiDAR detection distance and vehicle speed on the density of point cloud, the resolution of each measurement point in vehicle moving direction and LiDAR scanning direction was calculated, and the variable-scale grid was established for MLS point cloud data. The grid area was taken as the leaf area covered by laser beam, and the regression model of real total leaf area (TLA) and total grid area (TGA) was established. In the experiment, simulation tree was used to simulate the target of thinning fruit trees, and an MLS measurement system was built to collect the cloud data of the target points. Through changing the detection distance and moving speed, totally 108 sample data for four targets with different thinning degrees were obtained. The experimental results showed that the number of target cloud was decreased significantly with the increase of detection distance and the decrease of moving speed with variation coefficient over 0.9209, while the area of target grid can be extracted stably with variation coefficient less than 0.0537. The r-squared of linear fitting of TLA and TGA was 0.9090, and the mean value of relative error of TLA measurement was 9.16%.