With the analytic method of the construction-mechabism-function the authors have analysed the ball-like motion of intervertebral disc,and,especially,have calculate its motional status in biomechanics at the conjugative point,obtainning the characteristic values of ball-like conjugative motion of the supportive point of intervertebral disc,and,further,contraryly researching into the mechanism of intervertebal disc protrusion and its contrary characteristic value.