With the increasingly strict national requirements for nitrogen oxide (NOx) emissions,improving the level of NOx control has become a research focus in environmental protection field.The NOx generation mechanism of boilers was summarized,the characteristics of low-NOx combustion technologies was analyzed,mechanism characteristics and appliation of oxy-fuel combusiton was discussed. The results show that NOx formation mechanism is quite complex,which is closely related to fuel characteristics,oxygen content,nitrogen content,temperature,boiler structure and so on. Based on the different formation mechanism,the NOx produced in boilers can be divided into five types：fuel-NOx,thermal-NOx,prompt-NOx,NNH route and N2O-intermediate route. Different low-NOx combustion technologies or their combination can be applied to reduce NOx emission of different boilers. Low-NOx combustion technology has the advantages of diversified types,wide adaptability and low cost,which is one of the important technologies to realize NOx emission reduction. At present,the widely used low-NOx combustion technologies include air staged combustion technology,fuel staged combustion technology and flue gas recirculation technology. A wealth of operation experience has been accumulated for air staged combustion technology,which is suitable for boiler renovation projects,but the NOx emission reduction effect is limited and the problems of inadequate combustion and corrosion cannot be ignored.For fuel staged combustion technology,the NOx emission reduction effect is significant when gas fuels represented by natural gas are used as reburning fuels,and it is more suitable for new boilers. It is easy to retrofit the existing boilers for flue gas recirculation technology,but the combustion stability and efficiency is not very good. The NOx emission reduction effect of flue gas recirculation technology is so limited when used alone that it should be combined with other technologies simultaneously. Nevertheless,the ultra low NOx emission can be realized by emerging technologies such as MILD combustion,porous medium combustion,oxy-fuel combustion and MILD oxy-fuel combustion,thus having a very wide application prospect. MILD combustion technology has the advantages of good combustion stability,high combustion efficiency and low NOx emission. MILD combustion can be achieved for gas,liquid and powder solid fuels in both fully premixed and air unpreheating conditions. It also has significant advantages for the combustion of low calorific value fuels. Porous media combustion technology has the characteristics of high combustion efficiency,low NOx emission and wide lean combustion limit are,which can realize combustion of low calorific value gas fuels such as blast furnace gas,biomass gas and VOCs. As a clean combustion technology,oxyfuel combustion technology can achieve ultralow pollutant emissions and carbon capture at the same time,of which the economic and social benefits are of great worth. Combining the advantages of MILD combustion technology and oxy-fuel combustion technology,MILD oxy-fuel combustion is a new and efficient technology that can achieve near zero pollutant emissions for fossil fuels such as coal or natural gas.