In order to ensure the Electric Spring (ES) works within the effective operating range to achieve critical load voltage stability and power balance between supply and demand, the operating principle of ES for pure reactive power compensation is analyzed with the vector diagram method, the effective operating range of ES is obtained, and the effects of line impedance and other parameters on the range are discussed. An algorithm for adaptively adjusting the critical load voltage target value is proposed since expanding the effective operation range need to introduce active power compensation in existing methods. This algorithm can expand the effective operation range of ES only by reactive power compensation. The effect of the adjustment step length on the effective operation locus of line voltage is analyzed in detail. Finally, the simulation model in Simulink is established to validate the analysis results and the proposed algorithm. This work is supported by National Natural Science Foundation of China (No. 51467012).