There is no anti-slip brackets in the construction design of the VVER-1000 main coolant piping. Taking into account the nuclear safety of LBB, according to the requirements of NNSA, the nuclear safety level of the main pipeline has been increased from the originally designed Class B equipment to the Class A equipment management.Based on the regulations and the welding procedure qualification (WPQ) instruction (ППА) which were provided by Russian, an additional impact toughness requirement was added in the inspection items. The primary WPQ was failed because of part of impact values was too low in WPQ with 5G1T welding position.The article is focus on welding parameters,test values, welding materials, heat input etc. influence factors in WPQ with horizontal fixed position of pearlite heat-resistant steel, enhance of research through theory aspect.The relevant influencing factors were analyzed and demonstrated, and finally passed the process qualification test and verification, and the welding process qualification task was successfully completed. The actual application in the main coolant piping of the 3 and 4 units of a nuclear power plant has achieved good results. It has reference and promotion value for subsequent welding of the main coolant piping of similar type of nuclear power units and other similar pearlite piping with grand diameter and wall thickness.