The invention discloses a unit output optimizing method for improving transient stability margin of a receiving end power grid. According to the method, for the receiving end power grid with a known running state, the transient stability margin of each generator and the sensitivity of transient stability margin to the active output of the unit are calculated, the generator transient stability constraint is used as a constraint condition, a mathematical model of unit output optimization is proposed, and the optimal scheme of each generator output is obtained. According to the method, various operation constraint conditions in the output process of the optimization unit are comprehensively considered, and the output limit value, the limit value of the branch power and the limit value of thevoltage amplitude of a node are considered, so that the adaptability of various operation modes of the system is high, engineering practical operation is easy, and the transient stability characteristic of the generator can be ensured to meet the expected condition by optimizing the unit output.