The invention discloses a device and method for promoting nitrosation-anaerobic ammonia oxidation granulation through a UMABR reactor and belongs to the technical field of wastewater biological nitrogen removal. The device comprises a raw water tank, a UMABR reactor, an oxygen supply device, a gas collection device and an overflow bottle. A temperature control heating device is arranged in the raw water tank. The top of the UMABR reactor is provided with a glass funnel for separating gas and a liquid so that a biomass loss is reduced. The rear surface of the glass funnel gas separator is connected to the gas collection device. A gas pressure gauge and a needle valve for controlling an oxygen supply rate are arranged between the oxygen supply device and the reactor. The overflow bottle is arranged at a water outlet so that a part of outlet water passes through a reflux pump and is mixed with water. A DO, PH and ORP online monitoring device is arranged in the overflow bottle. The method comprises starting the UMABR reactor for promoting nitrosation-anaerobic ammonia oxidation granulation. The device and method are suitable for nitrosation-anaerobic ammonia oxidation biological denitrification of wastewater. The device has an advanced simple structure and has the advantages of energy saving and consumption reduction.