Objective: To explore the forces of lower limbs during side slip and forward slip through biomechanical study, and to provide a theoretical basis for the development of lower limb movements. Methods: BTS SMART DX motion capture analysis system, 8 infrared high-speed cameras, Kistler three-dimensional dynamometer, BTS FREEEMG 1000 portable wireless surface electromyography system were used to collect real-time data of side slip and forward slip. SPSS 20.0 univariate analysis of variance (ANOVA) was used to compare the differences of indicators between different groups. Results: In the side sliding movement, there were significant differences in hip motion (P<0.01) between 3-year-old group and 5-year-old group, hip motion(P<0.01) between 4-year-old group and 5-year-old group, knee motion(P<0.05) and ankle motion(P<0.05) between 3-year-old group and 5-year-old group. There was a significant difference in hip motion(P<0.05) between 3-year-old group and 5-year-old group. The Fx force(P<0.01) and vertical Fy force(P<0.01) in the left and right directions of the 3-year-old group and the 5-year-old group were significantly different in the side-slip movement, while the Fx force(P<0.01) in the right and left directions of the 3-year-old group and the 5-year-old group were significantly different in the forward-slip movement, and the vertical Fy force(P<0.05) was significantly different. There were significant differences in the long head of biceps femoris, gastrocnemius and anterior tibial muscle between 3-year-old group and 5-year-old group (P<0.05). There were significant differences in the middle gluteal muscle, gracilis muscle, adductor magnus, pubic muscle, medial femoral muscle and soleus muscle between 3-year-old group and 5-year-old group (P<0.05). There was significant difference in the anterior tibial muscle (P<0.05) between the 3-year-old group and the 5-year-old group in the supporting stage, and significant difference in the gastrocnemius muscle(P<0.05) during the swing stage. There was significant difference in the anterior tibial muscle(P<0.05) between the 3-year-old group and the 5-year-old group in the swing stage. Conclusion The development of side slip and forward slip in children aged 3 to 5 years old showed that the results suggest motor development increases significantly with age among preschoolers. Rhythmic movements of side slip and forward slip should be selected in the critical period of children’s movement development at different ages, laying the foundation for good physical and mental development.