A new interior permanent magnet synchronous motor (IPMSM) control method is presented for electrical vehicle drive system. Based on the differential geometry theory, the nonlinear system is changed into a linear system with input-output decoupling technique. Then, by the maximum torque per ampere (MTPA) control strategy, the output torque of the drive system is added. And then, a load torque observer is designed to resist the variable load. In the end, Based on the linearization model, an controller is given. Simulation results show that the electrical vehicle drive system based on decoupling robust control has fast transient responses, good load disturbance resistance responses and good tracking responses.