Abstract: The butt welding test of 100 μm thick TC4 titanium alloy ultra-thin plate was carried out by pulse micro-plasma arc welding (P-MPAW). The forming characteristics and microstructure of the welded joint were observed by microscope, and the welding process was simulated by finite element software. The welding mechanism of pulse micro-plasma arc welding of 100 μm thick titanium alloy ultra-thin plate was revealed. The results show that when the average current is 2 A, the duty cycle is 30% and 50%, when the pulse frequency is 25 Hz, the weld is fish scale weld, and the weld width is inversely proportional to the base current. When the pulse frequency is 100 Hz and 200 Hz, the weld shows a smooth continuous weld, and the weld width is inversely proportional to the pulse frequency. The microstructure of the weld zone of the welded joint is basketweave structure, and the heat affected zone is mainly equiaxed α phase, β phase and acicular α' phase martensite. Combined with the process test, it can be seen that the weld formation under different pulse parameters simulated by the finite element software is similar to the weld formation of the process test, and the influence of different base current and pulse frequency on the weld width is consistent.