Arc plasma is the source of heat and force for molten pool with dynamics fluid and convections. The arc force is studied with shape surveillance and force measurement during pulsed welding process. The results indicated that the average force increased by 42–57%. And the pressures were 322 Pa (conventional welding), 409 Pa and 517 Pa (pulsed welding). Imaging process is carried out for energy distribution analysis. The core region ratio increased up to 51%, compared with 43.7% by conventional welding. The arc pressure derivation was produced and the error was discussed. The increased arc force happens with arc constriction during pulsed welding. It is supposed to create surface depression of molten pool, enhance the fluidity and improve the heat convections.