Abstract: In order to analyze the factors which affect the coupling efficiency between LD and the semi-tapered multimode fiber,considering the multi-reflection on the tapered surface,using improved ray-tracing algorithm,the coupling efficiency of LD to the semi-tapered multimode fiber without cladding was calculated in different cases separately,such as different semi-tapered angle or different semi-tapered width,or different relative shift and incline.The results indicated that when semi-tapered width value was appropriate,the influence of the tapered angle on the coupling efficiency was unconspicuous;The coupling efficiency was sensitive to the relative shift,while it was inertial to the relative incline.