Abstract: Q550 steel thick plates were welded by CO₂ gas shielded flux cored welding and CO₂ gas shielded solid welding respectively at three heat inputs（13.0 kJ/cm, 15.0 kJ/cm and 18.0 kJ/cm）. The effects of heat input on the microstructure and mechanical property of the welded joints were studied. The results show that with the increase of heat input, the acicular ferrite content increases, the acicular ferrite grows up and the massive ferrite content decreases in the weld zone. Compared with solid welding, the metallurgical reaction products of the elements in the flux cored welding wire increase the nucleation points of acicular ferrite, so that the acicular ferrite in the flux cored welding seam is more and finer at the same heat input. With the increase of heat input, the elongation and the strength-plasticity of the joints welded with two kinds of CO₂ gas shielded increase. The mechanical property of the welded joints is optimal at the heat input（18.0 kJ/cm）. At the same heat input, the mechanical property of the flux cored welded joint is slightly better than that of solid welded joint. The mechanism of better mechanical property of Q550 steel welded joints is the increase of the fine acicular ferrite and the reduction of massive ferrite in the weld seam with the heat input increasing. It improves the mechanical property of the joints welded by flux cored welding more significantly.