A normal temperature low-ammonia-nitrogen nitrosation starting method belongs to the field of urban sewage treatment and reclamation. Researches show that nitrosation starting can be achieved in short time by controlling condition of low dissolved oxygen (DO=0.30mg/L) and inoculating sludge with certain nitrosation effect. If whole course nitration sludge is inoculated, no accumulation of nitrite occurs within 30 days in a water distribution test, no accumulation of nitrite occurs in 30 days in a reactor which utilizes the A/O dephosphorization process processing water as a research object, and the nitrosation starting fails accordingly. Nitrite accumulation can occur by adopting the mode of high-low gradient oxygen limit culture, domesticating the sludge for 10 days under the condition that DO=0.70-0.80mg/L, then controlling condition of low dissolved oxygen (0.30-0.40mg/L) to enable the reactor to discharge water if the whole course nitration sludge is inoculated. Over 90% of nitrosation rate can be achieved within 38 days by continuously domesticating the sludge under the condition of the dissolved oxygen, so that nitrosation can be successfully started.