Abstract: Dynamic variation mechanism of welding thermal cycles at different points in weld central line and influences of pulse parameters on it were studied for pulsed microplasma arc welding (P-MPAW) 100 μm ultra-thin stainless steel (SS) sheets. Combining experimental results about weld morphologies with different pulse parameters, the relations between weld morphologies and dynamic characters of welding thermal cycles were discussed further. The results were as following. There are five different weld morphologies obtained after welding with different combination of four pulse parameters. Welding thermal cycles tend to be identical at different points of one pulse cycle distance in weld center line. When the ratio of peak current to base current increases at low pulse frequency, the maximum values and minimum values in T_m and t_8/13 (heating time, i. e. time span for temperature rising from 800 ℃ to 1 300 ℃) at different points of one pulse cycle distance in weld center line decrease and increase respectively, and both maximum value and minimum value of t_8/5 increases. With an increasing in pulse frequency, the average values of t_8/5 and t_8/13 decrease. In most cases (when pulse frequency is not lower than 5 Hz or when base current are 0.5 A and 0.9 A at pulse frequency 1 Hz and 5 Hz, respectively), experimentally obtained duty cycles about weld morphologies (σ₁) were as the same as the calculated duty cycles about the highest temperature of welding thermal cycle (σ₂). It indicated that the calculated results were in agreement with the experimental results.