In the recognition of off-line Chinese char, thinning is an important step. Mathematical morphology algorithm is the most useful thinning method in recent years. The results are heavily influenced by the structuring element we used. With the large amount of the analysis of the thinning algorithm and the character of the Chinese char, we improved the previous structuring element. The objects are altered to unitary char. 23 group of Chinese char have been used to experiment and each group have 100 Chinese char. The results show that this algorithm solved the problem of losing the ending information and reduce the running time.