The invention discloses a method, a system and a device for optimizing an inter-regional day-ahead power transmission plan. The method comprises the steps of: taking the minimization of variance of aregional daily power generation sequence as an optimization object according to load forecast data of each region, intra-regional power balance and an inter-regional electric quantity plan, and establishing an objective function of an inter-regional day-ahead power transmission plan optimization model; and determining constraint conditions and model parameters of the inter-regional day-ahead powertransmission plan optimization model. The system comprises an objective function establishment module and a constraint condition and model parameter determination module. The device comprises a memory and a processor. According to the method, the system and the device, the minimization of variance of the regional daily power generation sequence is taken as the optimization object, thus peak shaving stress of power transmission regions is relieved through the minimum inter-regional power transmission and reception output adjustment, and the complementary optimization role of load features of each region in peak shaving of the power transmission regions is fully played, so that an inter-regional power transmission and reception plan is matched with a power transmission region peak shaving rate. The method, the system and the device can be widely applied to the field of power system scheduling optimization.