The invention discloses a power plant medium and long term limit transaction electric quantity optimization method based on SCUC. The method comprises the following steps of acquiring operation parameters and plan data of a power grid, wherein the operation parameters comprise line tide upper and lower limits, unit output upper and lower limits, unit minimum starting and stopping time and unit climbing speed, and the plan data comprise load requirements, standby requirements, maintenance plans and fixed output plans; constructing a safety constraint unit combination model according to the operation parameters and the plan data; constructing an optimization model of limit transaction electric quantity calculation according to the security constraint unit combination model; and solving themodel obtained in the previous step to obtain the medium-and-long-term limit transaction electric quantity of the power plant. According to the method, various factors for limiting the transaction electric quantity of the power plant can be fully considered, a more reasonable limit transaction electric quantity result of the power plant is finally obtained on the basis of the optimization model ofthe safety constraint unit combination, medium-and-long-term power grid operation risks after bilateral transaction scale increase are favorably pre-controlled, and the safety and the economy of power grid operation are improved.