The invention relates to the technical field of welding, and specifically discloses a core filling method for a solder wire soldering flux. The method comprises steps of positioning and placing a hollow tin tube before manufacturing, heating a soldering flux, heating a tin-tube heating barrel and a rosin heating barrel, preserving heat, filling core, and cooling, so as to wrap the soldering flux in tin alloys. The core filling method is simple in manufacturing, and would not disturb normal operation of a workshop production line, and is suitable for small-scale production of solder wire samples. The method improves production efficiency and reduces production cost.