Through the comparative analysis of several methods of on-site rail welding,it can be seen that the on-site flash butt welding has the highest emission. However,the high quality and high efficiency makes it occupy an irreplaceable dominant position in the field of rail welding. In this paper,the causes of serious emissions from on-site flash butt welding were analyzed:the generator power required for heating is very large and the liquid bridge formation and blasting during heating causes transient overload of the generator set;incomplete combustion of the generator creates harmful fume particulate emissions such as CO and etc.,this can be very harmful in the seamless railway construction,especially bring great harm in the construction of the seamless subway. Based on UN5-150 ZB series welding machine,analyzed the relationship between the commonly used welding process and generator set emissions,and further proposed a new emission reduction technology strategy considering welding process parameters such as welding current,voltage,feeding speed,retreat speed and live upset time and etc. Through the process test,we obtained the process parameters that satisfying the requirements of TBT1632-2014,and this greatly improved the subway construction environment. In addition,new attachments to reduce emissions for generator sets were also introduced.