Abstract: This paper analyzes that temperature variation affects unjunction transistor phase shift trigger circuit of silicon controlled rectifier (SCR) welder by PROTEL99-SIM simulation software.And the results show that temperature drift of the unijunction transistor deeply affects the phase shift time, thus affects the stability of welder's outputs.On the other hand, digital welding machine now becomes more and more popular, which demands welder's digital trigger.The study is based on the digital signal process(DSP_S),taken use of the software to produce trigger in order to control the main circuit of welding machine.The study shows that the trigger from DSP is credibale and the software is comparatively simple.All of these make complex shift trigger circuit into software program.It brings welding machine into true digital control age.