A type of groove targeted on ultra- narrow- gap multi- pass welding is designed and 50 mm thick 30Cr2Ni4MoV turbine rotor steel is welded by CO2 laser welding with filler wire based on the groove. After welding process, the microstructures of different zones of joint are observed and analyzed. The tensile test, bending test and measurement of microhardness are conducted to evaluate the properties of joint. The results show that the joint of 50 mm thick 30Cr2Ni4MoV turbine rotor steel has fine appearance without defect under the optimized parameters of ultra- narrow- gap multi- pass laser welding with filler wire. The middle zone of weld consists of a mass of acicular ferrite and a few of granular bainite and it shows good impact toughness. The surface layer of weld consists of martensite and a few of granular bainite and its impact toughness decreases a little. The fractures occurre on the base metal of tensile test specimens. The peak value of microhardness is achieved in the coarse grain zone in heat affected zone (HAZ) of surface layer of weld and the microhardness in weld is much lower than that in HAZ.