Radar Jamming Cancellation system and some adaptive filtering algorithms are studied. Some variable step size LMS algorithms in literature are analyzed, based on which an improved one is presented. For this algorithm the similar sigmoid function is used to adjust the step size, thus the quantity of operation is greatly reduced and in the similar sigmoid function the self-correlation value of the error signal is used to adjust the step size, so the contradiction between the convergence speed and steady-state error is solved. At the same time, the use of LMS algorithm decreases the noise sensitivity. The computer simulation results confirm the theoretical analysis and show that the algorithm is of some feasibility and superiority.