From magnetic field equation and the principle of the work and energy, We derive the effect of the magnetic field gradient and magnetic flux invariance. Further, we obtain the axial magnetic field intensity at the retrace point of the charged particle in magnetic constrain device. 根据磁场方程和功能原理,导出了磁场梯度效应,并得到磁通不变原理,从而找到,在磁约束装置中,带电粒子折返点处的轴向场强。