The invention provides a power output control method, device and system for a generator. The method comprises the following steps: collecting a high-voltage DC busbar voltage in a preset control period in real time, and calculating an actual voltage value of a busbar; calculating a busbar voltage deviation according to the calculated actual voltage value of the busbar and a preset busbar voltage set value; calculating a torque increment through an increment amplitude-limiting adjustment method according to the calculated busbar voltage deviation; dynamically superposing the calculated torque increment, and preset upper torque limit value and lower torque limit value in each control period according to a preset initial torque value, and calculating the given torque of the generator; adjusting the given torque of the generator and controlling the power output of the generator according to the calculated given torque under the condition keeping the rotating speed of the generator constant.