Abstract: While Cuk DC-DC converter is widely researched and applied, especially as new energy converter, it‘s more and more urgent to improve the efficiency with simple control method. The Cuk symmetry circuit structure is easy to achieve bidirectional converter with higher power density and low ripple rate. By the simulation with Matlab/Simulink, this paper presents L×C characteristic of the Cuk circuit with which the circuit‘s effective and stable operating range of(fs,D)can be achieved and its efficiency, operating frequency, output voltage, ripple rate and output power can be designed and regulated effectively and concisely. Then, one presents an effective modulation method of the converter with a constant L×C values as the load changing up or down, and so higher efficiency can be kept. All this is a reference to the DC-DC converter efficiency control scheme choice in practical application.