Projection welding is a variation of electric resistance welding with the dynamic changes of the flow paths for heat and electrical properties with changing temperature caused by the large plastic deformation with collapse of projection. The common standards of projection welding are AWS, IIW and Harris& Riley which have been established and in use for some time. All these standards have the slots located under the projection. However, the standard proposed in this paper researches on is a special one with the slots designed on the two sides of the projection. A comprehensive thermal-electrical-mechanical analysis was performed to quantitatively simulate the projection welding of the special standard by a coupled finite element method. The electrical, thermal and mechanical aspects of the projection welding processes such as the projection collapse mechanism, the nugget formation process and the effects of different electrode forces were discussed in this paper.