An engine room wind speed and power curve based method for evaluating theoretical power of a wind farm. The method comprises the following steps of: determining measurement data of an engine room wind speed of a wind turbine generator and an actual output power of the wind turbine generator within a normal operation period of the wind turbine generator; determining a wind speed and power curve of the engine room wind speed and the actual output power within the normal operation period of the wind turbine generator; determining theoretical power of the wind turbine generator within an abnormal operation period of the wind turbine generator; and determining the theoretical power of the wind farm within the abnormal operation period of the wind turbine generator. According to the method, a wind tower does not need to be set up, so that operation and maintenance cost of the wind farm can be greatly reduced; errors introduced due to an excessively long distance and improper selection of parameters or methods in the flow field calculation process are avoided and estimation accuracy is improved; and due to refinement of calculation to the single fan degree, the method is convenient to distinguish whether the source of electricity loss is scheduling abandoned wind or a fan failure and the like.