Abstract: Aiming at the problem that normal frequency converting system couldn’t do frequency converting directly to middle-voltage and high-voltage motors with lager capacity of coal mine,the paper introduced design of a high-high direct frequency converting system.It gave structure and working principle of the system,introduced design and simulation of rectification circuit and inverter circuit in details and analyzed key technology of the system design.The system has simple structure,low cost,small size and easy operation.Actual application showed that the system can save energy effectively and has high reliability,fast response speed,flexible control and small output harmonic with strong field practicability and adaptability.