Objective　To explore the function of psoas major muscles on lumbar spine movements in biomechanics of Rhesus.　 Methods　Four Rhesus were studied in our study. The dynamic AP and lateral digital X-ray views of Rhesus were conducted in three different status, including normal status, one side psoas major resection and bilateral psoas major resection. Vertebral rotations of the spine were evaluated in AP views. The segment areas of lumbar curvatures were calculated in lateral views in applied mathematics geometry. The parameters of vertebral rotational degrees and segment areas of lumbar curvatures were analyzed in three different positions: sitting, standing and stepping in statistics.　Results　For measurements of both the vertebral rotational degrees and segment areas of lumbar curvatures, there were significant differences between normal status and one side psoas major resection, but there were no significant differences between normal status and bilateral psoas major resection.　Conclusions　Psoas majors are the major muscles to maintain lumbar curvature and conduct vertebral movements. In statistics, these particular muscles are the anterior powers to maintain the lumbar spine balance.