The invention discloses a method for overlap welding of steel bars by adopting flux-cored CO2 gas shielded welding wires. The method is characterized in that an inverter type CO2 gas shielded welding machine is utilized as welding equipment, the ER501T-1 CO2 flux-cored gas shielded welding wires are adopted, and the steel bars are HRB335 ribbed steel bars with phi being 20mm-30mm. The method comprises the following steps that 1) the steel bars are overlapped, and spot welding is conducted on the overlapped steel bars; 2) root-layer welding is conducted on the overlapped weld joints of the steel bars, wherein an electric arc moves linearly, as for welding parameters, a welding current is 260A, a welding voltage is 23V, and welding speed is 35-40cm / min; 3) covering-layer welding is conducted on the root-layer weld joints, the electric arc moves in a zigzag mode, as for welding parameters, a welding current is 260A, a welding voltage is 23V, and welding speed is 25-35cm / min. The method can increase the welding speed, enhance rust resisting capability of the weld joints, prevent pore defects, and achieve the fact that cleaning does not need to be conducted in a welding process.