An economic operation method of an engine having servo control system is provided. The servo control system includes servo devices of two permanent magnet motors, a main control unit (13), an engine control unit (16) and an energy storage unit (15). The method includes the steps as follow: 1) the servo control system acquires current driving requirement power and charging requirement power to obtain total requirement power; 2) the main control unit (13) controls one of first and second motors or both of them to operate as a power generator when total requirement power is between lower limit value and upper limit value of economic operation range; 3) the servo control system controls the engine to operate at upper limit value of economic operation range when total requirement power is greater than upper limit value of economic operation range; 4) the servo control system switches off the engine and drives the hybrid vehicle by the electromotor only when total requirement power is less than lower limit value of economic operation range.