LF (ladle furnace)refining is a key metallurgical process for steelmaking cold rolled low carbon steel in BOF-LF-CSP flow sheet. The electric characteristics of a 120 t LF with 20 MW transformer and the power supply curves have been studied and optimized by established optimizing model to decrease refining time and improve refining effect. Results show that with different voltage level the maximum heating up of LF at Handan Steel is from 1.45 ℃/min to 5.14 ℃/min;and with third voltage level(302 V)the heating up speed shall be increased from 4.01 ℃/min at present to4.58 ℃/min.