A method for integratedly treating domestic wastewater and organic garbage comprises steps of multiphase separation, fermentation, biogas production and biological denitrification and dephosphorization, and it can translate domestic wastewater and organic garbage into clear water, biogas, organic fertilizer and sand residue, which can be discharged directly. An apparatus for integratedly treating domestic wastewater and organic garbage is also provided, which comprises a multiphase separation device, a fermentation device, a biogas production device and a biological denitrification and dephosphorization device. A multiphase separation device for separating domestic wastewater is also provided, which comprises a domestic wastewater inlet, a sand separation chamber, a sand outlet, an automatic separation screen device comprised of an abnormity screen and a separation rake, a screen isolate outlet, a floating liquid chamber and a supernatant chamber. A method for separating domestic wastewater using the multiphase separation device is also provided. Domestic wastewater and organic garbage can be treated, simultaneously, so as to realize an object of minimum pollutant discharge.