The soliton calculation method put forward by Zabusky and Kruskai has played an important role in the development of soliton theory,however numerous numerical results show that even though the parameters satisfy the linear stability condition,nonlinear instability will also occur.We notice an exception in the numerical calculation of soliton,gain the finear stability condition of the second order Leap-frog scheme constructed by Zabusky and Kruskal,and then draw the perturbed equation with the finite difference method.Also,we solve the symmetry group of the KdV equation with the knowledge of the invariance of Lie symmetry group and then discuss whether the perturbed equation and the conservation law keep the corresponding symmetry.The conservation law of KdV equation satisfies the scaling transformation,while the perturbed equation does not satisfy the Galilean invariance condition and the scaling invariance condition.It is demonstrated that the numerical simulation destroy some physical characteristics of the original KdV equation.The nonlinear instability in the calculation of solitons is related to the breaking of symmetry.