Objective To explore the effect of different intensity treadmill training on the repair of micro-injured Achilles tendon induced by collagenase in rats. Methods Seventy-two 8-week-old male Sprague Dawley rats (weighing, 200-250 g) were selected. After adaptive treadmill training for 1 week, rats were injected with 30 μL type I collagenase solution (10 mg/mL) into both Achilles tendons to make micro-injured Achilles tendon models. After 1 week of cage feeding, the rats were randomly divided into 3 groups: the control group, the low-intensity group, and the high-intensity group, 24 rats each group. The rats in control group could move freely, and the rats underwent daily treadmill training at the intensity of 13 m/min and 20 min/d in the low-intensity group and at the intensity of 17 m/min and 60 min/d in the high-intensity group. At immediate, 1 week, and 4 weeks after training, bilateral Achilles tendons were collected from 8 rats of each group for gross observation, histological analysis, and mechanical testing. Results At immediate after training, there was no significant difference in the gross observation, histological observation, and biomechanical properties of the Achilles tendon between groups (P>0.05). The gross observation showed connective tissue hyperplasia near Achilles tendon and lackluster tendon in each group at 1 week; hyperplasia significantly reduced in the low-intensity group when compared with the control group, and there were more connective tissue and a large number of neovascularization in the high-intensity group at 4 weeks. At 1 week, there was no significant difference in the semi-quantitative histological total score between groups (P>0.05), but there were significant differences in vascularity between low-intensity group or high-intensity group and control group (P<0.05). At 4 weeks, the semi-quantitative histological total score was significantly higher in high-intensity group than control group and low-intensity group (P<0.05), and in control group than low-intensity group (P<0.05). There were significant differences in collagen arrangement, cell morphology, abnormal cells, and vascularity between low-intensity group and high-intensity group or control group (P<0.05). And there was significant difference in abnormal cells between high-intensity group and control group (P<0.05). The mechanical testing showed that there was no significant difference in cross-sectional area of the Achilles tendon, the ultimate force, tensile strength, and elastic modulus between groups at 1 week (P>0.05); the low-intensity group was significantly higher than the control group in the ultimate force and the tensile strength (P<0.05), and than high-intensity group in the ultimate force and elastic modulus (P<0.05), but no significant difference was found in the other indexes between groups (P>0.05) at 4 weeks. Conclusion Low-intensity treadmill training can promote the repair of rat micro-injured Achilles tendon induced by collagenase.