The invention relates to an electric servo brake device of an electric car. The electric servo brake device comprises a control system, a brake system and a rotary servo motor (6), wherein the brake system comprises a brake master cylinder (1) and a brake pedal (2); the control system comprises a one-way clutch (3), a pedal rotary angle sensor (4), a pedal force torque sensor (5) and a controller (7). The electric servo brake device has the advantages that the electric servo brake device is provided with the one-way clutch, and a driver can complete car braking through human power when circuit failure occurs; the wedge type one-way clutch and the rotary motor with a reducer are mounted on the fulcrum rotary shaft of the brake pedal, and a power assisting effect on the brake pedal is achieved by only one motor; the electric servo brake device is used for the active braking of the electric car, and the default position of the brake pedal is in a brake state to guarantee safety; an intelligent driving module transmits a signal to the brake rotary servo motor, the brake servo motor rotates at a given angle according to a given angular speed and keeps the angle, and the motor continues rotating when brake force needs to be increased.