In order to improve the power quality of critical load(CL) and ensure the safe and stable operation of ES, a non-linear control method based on flatness-based control(FBC) is proposed to solve the problems of multivariable and strong non-linearity of electric spring(ES) system and narrow stability region and weak robustness of traditional linear PI control. Firstly, the dynamic mathematical model of three-phase ES (TPES) is established and the reference value of the voltage injected by TPES to stabilize CL voltage is calculated; then, the differential smoothing theory is introduced to verify that the TPES system is a differential smoothing system, and a differential smoothing non-linear controller based on TPES is designed to achieve high power quality of CL and safe and stable operation of TPES. Finally, the simulation results based on MATLAB/simulink show that the proposed differential smoothing nonlinear control method has the characteristics of fast dynamic response, high stability and strong robustness.