The invention discloses a method for welding a cast steel piece and low-alloy structural steel at low temperature. The method comprises the following steps: drying a welding material through a drying box, and then keeping the temperature of the welding material temporarily through a thermostat; opening grooves on a Q345D parent metal and a ZG230-450H parent metal respectively; preheating the parent metals to a temperature of between 110 and 130 DEG C in the working environment that the temperature is between 5 DEG C below zero and 15 DEG C below zero; carrying out positioning welding, CO2 gas protecting and back welding, manual filling welding and manual overlay welding on the grooves among the parent metals sequentially, and keeping the temperature of between 180 and 200 DEG C after welding each welding layer; carrying out postweld heat treatment after finishing the welding; and heating the parent metals evenly after finishing the heat treatment, and keeping the temperature of a welding seam after the temperature reaches 250 to 300 DEG C. The method can prevent joint embrittling or cold cracks generated from the surface and make the impact toughness of all joint areas reach a minimum value of the parent metals, so as to greatly shorten the construction period.