The pulse TIG welding with low frequency and large duty cycle was used to realize single-side welding without subtrate and double-side forming of 16MnR steel plates with 5mm large blunt edge，in view of the low effi- ciency，complex process and poor stability in traditional manual welding with subtract for medium and thick plates backing welding． The compression process of welding arc passing through the molten hole was captured by a high- speed camera． And the correlation between deflection angle and length of arc tail flame and weld penetration at a certain welding speed was analyzed． The results show that a hole-shaped weld pool is formed under a certain para- meters，and the deflection angle and length of the arc tail flame passing through the hole is negatively correlated with the welding speed，and positively correlated with the weld penetration． By observing the size of the arc tail flame，the weld penetration can be estimated，and the stirring and convection of liquid in weld pool is enhanced un- der the thermo-mechanical coupling effect of periodic arc． Under the combined action of surface tension，gravity， arc force and gas blowing force，the liquid metal on the side wall of the welding hole flows to the edge and bottom， which enhances the heat conduction in the weld pool and effectively increases the weld penetration．