The invention discloses a denitrifying and dephosphorizing composite device and a denitrifying and dephosphorizing method thereof. The denitrifying and dephosphorizing composite device comprises a coagulating settling tank, a flow regulating tank, an anaerobic tank, an anoxic tank, a sequencing batch reaction tank, treated water tanks and nitrified solution transferring tanks, wherein the anaerobic tank is arranged inside the anoxic tank; the anoxic tank is arranged inside the sequencing batch reaction tank; the two ends of the anaerobic tank are open; the bottom end of the anaerobic tank is communicated with the lower part of the anoxic tank; the top end of the anoxic tank is open, and is communicated with the sequencing batch reaction tank through a sewage overflowing zone; and the nitrified solution transferring tanks are arranged at the upper part of the sequencing batch reaction tank. The denitrifying and dephosphorizing composite device is unique in design and small in occupied area; an anaerobic zone, an anoxic zone and an aerobic zone are communicated with one another, and water inletting, stirring and aeration are performed at regular intervals; during water inletting, a whole system switches among anaerobic treatment, anoxic treatment and aerobic treatment, and when water inletting stops, the anaerobic zone, the anoxic zone and the aerobic zone perform own microbial reactions, without mutual interference, so that forward dephosphorization and reverse denitrification of wastewater are performed circularly and steadily; therefore, the denitrifying and dephosphorizing composite device is low in running cost, and is suitable for popularization.