The conventional postweld heat treatment（PWHT） can not be applied to the key equipment of nuclear power plant after repair welding during service. In order to solve this problem,a mechanized gas tungsten arc welding（GTAW） is used for temper bead welding procedure test of the low alloy steel for reactor pressure vessel under the preheating conditions at room temperature and 150 ℃ respectively,and then the microhardness,microstructure and impact toughness of the heat affected zone（ HAZ） are tested. The results show that the microhardness can be effectively reduced by properly controlling the heat input ratios of the first three layers,and the tempered sorbite structure with good comprehensive properties can be obtained in the HAZ. Both the impact absorption energies at 0 ℃ and 20 ℃ of the HAZ of the temper bead under the preheating conditions at room temperature are higher than the design requirements.