The invention discloses a one-piece flat-plate solar photo-thermal photovoltaic system. The one-piece flat-plate solar photo-thermal photovoltaic system comprises a flat-plate solar heat collector, a solar photovoltaic power generating panel, a water tank, a circulating water pump, a storage battery, an inverter and a controller, wherein the flat-plate solar heat collector is provided with a tubular heat absorber, a water inlet of the tubular heat absorber is connected with the water tank through a first pipeline, and an outlet of the tubular heat exchanger is connected with the water tank through a second pipeline; and the circulating water pump is arranged on the first pipeline and electrically connected with the controller. The one-piece flat-plate solar photo-thermal photovoltaic system further comprises a thermoelectric power generating system, wherein the thermoelectric power generating system comprises a cold water circulating system, a third pipeline connected between the first pipeline and the second pipeline and a thermoelectric power generating fin; and the hot end of the thermoelectric power generating fin is connected with the third pipeline, and the cold end of the thermoelectric power generating fin is connected with the cold water circulating system. The one-piece flat-plate solar photo-thermal photovoltaic system has high solar utilizing rate and can meet the requirements of users on hot water in rainy days.