A method and a device for detecting an active power of a wind turbine. The method includes: acquiring a current rotation speed of a rotor of a wind turbine and a current outputted active power; determining an effective wind speed of the wind turbine; determining a maximum active power capable to be captured by the wind turbine at the current rotation speed; determining a maximum active power capable to be outputted by the wind turbine, and determining a release power at which the wind turbine is capable to release a rotation kinetic energy of the wind turbine for a predetermined time at the current rotation speed; and determining the available active power of the wind turbine, based on the maximum active power capable to be outputted, the release power, and the current outputted active power.