The invention relates to the technical field of cascade water and electricity combined optimization, in particular to a day-ahead cascade water and electricity combined optimization method and device.The method comprises the steps of obtaining a first model based on annual adjustment and hydropower station control as target modeling; based on the seasonal regulation hydropower station control target modeling, obtaining a second model; based on daily regulation hydropower station control target modeling, obtaining a third model; and modeling based on the first model, the first model and the first model by taking cascade hydropower station joint optimization control as a target to obtain a fourth model. Carrying out cascade upstream and downstream flow coordination constraint based on a fourth model; performing single hydropower station operation characteristic constraint based on a fourth model; performing cascade hydropower station operation characteristic constraint based on a fourthmodel; and solving the fourth model after the cascade upstream and downstream flow coordination constraint, the single hydropower station operation characteristic constraint and the cascade hydropower station operation characteristic constraint to obtain a cascade hydropower combined optimization scheme.