In order to realize the narrow-gap welding of 20 mm thick plate 16MnDR steel, experiments are designed to study the effect of root faces and groove angles on welding process stability, weld appearances and welding efficiency, then optimize the size of the groove, on this basis, using autogenous laser as bottom welding, multi-pass multi-layer laser-MIG hybrid welding to fill the gap. The results indicate that the weld appearance is pretty good and there is no defects of internal weld. Comparing the welding heat input and welding efficiency of laser-MIG hybrid welding with traditional arc welding, the welding heat input of laser-MIG hybrid welding decreases by 45%, and the welding efficiency of laser-MIG hybrid welding increases by 155%. The tensile strength of the joint is slightly higher than that of the base metal, satisfying the strength requirement, and there is no crack while the joints are bent to 180°, meeting the yield strength.