Objective To compare the biomechanical characteristics of lower limbs of young people with different body mass index (BMI) before and after the intervention of Baduanjin exercise, so as to provide references for scientific exercise and sport prescription for young Baduanjin practitioners with different BMIs. Methods Young volunteers with different BMI were divided into standard group, overweight group and obesity group. The BTS SMART 3D infrared motion capture system was used to capture the motion parameters and electromyography (EMG) parameters, the KISTLER dynamometer was used to collect the dynamic parameters, the AnyBody simulation software was used to calculate the kinematic parameters and dynamic parameters, and BTS SMART Analyzer was used to compare the different integral electromyography values. Results There was no significant difference in each parameter before the intervention of Baduanjin. After the intervention, the angle and angular velocity of the knee joint during extension and flexion in standard group, overweight group and obesity group, and the angular acceleration of the knee joint during extension and flexion in standard group significantly increased. Baduanjin significantly increased the knee flexion force and moment in standard group, overweight group and obesity group, and increased the hip flexion force and moment as well as ankle metatarsal flexion force and moment in standard group. Compared with obesity group, the knee flexion force and moment in standard group were significantly different. The knee joint force and moment were positively correlated with BMI. Baduanjin significantly increased muscle strength of iliopsoas, gluteus medius, piriformis, gluteus maximus, rectus femoris, sartorius, tibialis anterior, biceps femoris longhead, flexor digitorum longus, flexor pollicis longus, musculus peroneus longus and musculus peroneus brevis. Baduanjin significantly increased the integrated electromyogram (iEMG） of rectus femoris, tibialis anterior, biceps femoris. Conclusions Baduanjin can improve muscle strength and neuromuscular control ability of young people. The larger the BMI, the greater the flexion force and moment of the knee joint.