Abstract: Negative cam shedding mechanism is widely used in water jet looms and air jet looms.To reasonably select processing parameters,increase the life-span of attachments,decrease the depletions of energy and materials,reduce woven defect and improve quality,the author has established the calculating formula for the primary elongation and number of return spring of negative cam shedding mechanism based on its operating principle and mechanical analysis of its components,and discussed the factors that affect the primary elongation and number of return spring.The research findings revealed that it will be helpful for reducing the elongation distortion and enhancing the clearness of shedding with lesser tension, higher back,decrease of shed height,increase of shed length,reasonable arrangement of warp number on every heald frame and reduction of warp rigid coefficient.