In the process of directly starting a contactless transmission system, the peak power and current of the power input can reach several to hundreds of times the steady-state value. The converter switching tube is prone to a single hard switching phenomenon, leading to aging or even breakdown of the switching tube. In response to this problem, a three-phase LCC contactless power transmission system is taken as the research object. The homogeneous solution and inhomogeneous equation special solution, and complete solution of its equivalent circuit corresponding to the differential equation are derived. Given that the homogeneous solution is not directly related to the power source excitation and determines the transient characteristics, an ultra-low voltage soft start method is proposed. Analysis shows that: the theoretical basis of the soft start method is that the zero input response and zero state response of the differential equation have decomposable characteristics; mathematical modeling and simulation are used to verify that soft start can reduce the peak zero input response of the system; the circuit simulation verifies that soft start can effectively avoid the phenomenon of a single hard switch in the switching tube; if the power supply is cut off for a short time during the operation of the circuit and the driving circuit continues to work because of internal energy storage, a soft start method should be used to restart to avoid the impact caused by direct starting; an approximate calculation method for the initial rate of change is proposed, and the conclusion is that the initial value is related to power excitation. The experiment has proved the correctness of the above conclusion.