In order to improve the measuring accuracy of tree’s diameter at breast height (DBH) and decrease the working load, an algorithm for measuring tree’s DBH was proposed by combining the optical triangular method, image processing and least square fitting. The algorithm was proposed based on a series of theory analysis, and the measuring system was built up. Five trees with different diameters at breast height were measured and calculated. The testing result showed that the optical triangular method had higher accuracy, the error was less than 22mm, and the relative error was less than 5.5%.