Weak perspective model of camera was adopted. First of all, image of calibration target was captured to obtain pixel coordinates of 48 target centroids. Then hidden parameters matrix of camera was calculated by using least square based on DLT. Location tests of ordered and disordered weeds in indoor soil trough showed that positioning errors of ordered and disordered weeds were 19.2mm and 22.8mm respectively by using established hidden parameters matrix of camera. The positioning accuracy could meet the requirement of precise spraying of herbicide. 