Through computer simulation plus pilot tests,study is conducted in this paper for the mechanism of waste gas recycling technology influencing combustion process in the combustion chamber.Comparison is also made for the changes of NO produced in vertical flues under different amounts of waste gas to be added.This technology could help reduce the O2 content in the combustion air and minimize the NO formation at high-temperature position in the flues,i.e.when the amount of waste gas to be mixed reaches up to 40%,the NO in the waste gas could be cut by about 70%.