Abstract: [Objective] In order to improve the rate of the heterotrophic nitrification, we screened and identified a high-efficient heterotrophic nitrifier, as well as studied its nitrification characteristics and nitrification conditions.[Methods] We obtained activated sludge samples from sewage and chemical fertilizer factories and farmland. We then utilized sodium citrate and ammonium chloride as carbon and nitrogen source. We used methods including domestication, gradient dilution of domestication liquid, isolation from streaking plate and color indicator as rapid nitrification detection. Finally a high-efficient heterotrophic nitrifier was obtained. We identified this strain according to its physiological, biochemical properties and the sequence analysis of 16S rDNA. After inoculating the strain into artificial ammonia-nitrogen wastewater, changes of nitrogen compounds were measured in order to understand the nitrification characteristics. Nitrification condition was also optimized by changing the carbon source, dissolved oxygen, C/N ratio, temperature and pH of the medium. [Results] The heterotrophic nitrifier was a gram-negative bacilli. It neither fermented glucose, nor produced indole. Oxidase and catalase tests were positive. It could produce alkali if organic salt was provided. The strain shared 99.7% sequence identity of its 16S rDNA with ES-SDK-3 of Alcaligenes sp. In the artificial wastewater with 182.30 mg/L ammonia nitrogen as initial concentration, the removal efficiency by the strain was 99.8% after 30h cultivation. The average nitrogen removal rate was 9.61mg-N/L/h in its exponential phase. It produced almost no NO2―-N and NO3―-N in the entire nitrification process. The optimal carbon source is sodium citrate. Higher dissolved oxygen and C/N ratio favor its nitrification. When temperature is ranged from 30℃ to 35℃ and pH is ranged from 5.0 to 9.0, it can completely remove ammonia nitrogen. [Conclusions] The strain was identified as Alcaligenes genus, and named as Alcaligenes sp. HN-S. Our research confirmed that the Alcaligenes sp. HN-S had significant advantages over heterotrophic nitrifiers that were screened previously with aspect of ammonia nitrogen removal rate. The research of its nitrification condition definitely provided necessary theory support for a new biology process to remove nitrogen with high efficiency.