A novel prescribed-time second-order sliding mode(SOSM) controller is proposed for uncertain nonlinear systems. Firstly, by selecting the new sliding mode variable, the standard SOSM integral chain system is extended to a new SOSM dynamic system with mismatched terms, and the use of the new sliding mode system can effectively reduce the uncertainties in input channel. Secondly, by using the concept of time-varying scaling function, a prescribed-time SOSM control algorithm based on switching strategy is proposed for the new sliding mode system when the upper limit of the gain function of the mismatched item is unknown. The proposed controller switches from a finite time domain SOSM controller to the traditional finite time SOSM controller, the former can ensure that the sliding mode variable converges to zero at a prescribed time in the finite time domain, and the convergence time does not depend on the initial conditions of the system, and the latter can ensure that the sliding mode variable remains at zero thereafter. Finally, the prescribed time stability of the closed-loop system is analyzed. The theoretical results are verified by simulation comparison.