The present invention discloses a spacecraft solar wing in-orbit full life cycle output power prediction method and system. The method comprises the steps of: calculating in-orbit solar wing output feature over-all attenuation factors of each moment according to telemetry data of an in-orbit output short-circuit current and an open-circuit voltage and ground calibration data, and performing fitting to obtain a changing curve of the over-all attenuation factors with time; and for an in-orbit random moment t to be predicted, calculating a solar wing output feature according to a real-time solarincident angle, an in-orbit operating temperature, an earth-sun distance and two attenuation factors FI(t) and FV(t) at a t moment in the changing curve to predict a solar wing in-orbit random momentoutput power. The spacecraft solar wing in-orbit full life cycle output power prediction method and system overcome defects that a traditional estimation method only can calculate a solar wing outputpower at a life last phase and can effectively reflect real performances of the solar wing in-orbit.