Abstract: Electrical beam control system usually used on warp knitting is divided into two types: the closed loop control system and semi-closed loop control system.As to the semi-closed loop control system,the precision of yarn let-off is inferior,and it influences the quality of products.The paper analyzes the discrepancy of the working principle between these two system,as well as the factor that influences the precision of yarn let-off,and detect the yarn let-off deviation of these two system.It is found that the closed loop control system has better control precision than the semi-closed loop control system,however,provided that good warping is effected,the semi-closed loop control system which is easy to manipulate can also offer perfect control precision.