In a conventional voltage type superconducting magnetic energy storage system（VSMES）， the voltage sourced converter（VSC） and the chopper are controlled independently， which fails to consider the mutual influence of the two parts and therefore is poor in compatibility. A complete mathematical model of the VSMES is established and a unified nonlinear control strategy based on Lyapunov direct method is also proposed for the VSMES， which significantly improves the system stability under large signal disturbances. Furthermore， the dc link capacitor volume can be reduced because the VSC and chopper are controlled harmonically as a whole. In the pulse width modulation（PWM） control of the chopper， the phase-shift PWM method is adopted， which doubles the equivalent switching frequency and has the benefit of even power distribution among switches. Computer simulations verify the effectiveness of the proposed control strategy.