Laser-MAG hybrid welding technology was used to weld the non-groove T-joint S355 J2 W bogie frame with 12 mm thickness in order to obtain the welding by one side with back formation joints. The focus location offset along the two directions due to Tool Center Point（TCP） change in automatic welding was described,which are defocus offset and the offset along vertical plate surface of T-joint. According to the test results of the appearance and metallographic examinations,the influence of offset along the above two directions on welding quality is discussed. The maximum allowable offset ranges were obtained,which are-2.5,-,1.5 mm of defocus and1,2, mm along vertical plate surface. In this range,a stable welding by one side with back formation can be achieved.