The magnetically controlled narrow-gap welding technology was used to weld the 30 mm and 100 mm thick TC4 titanium alloy. Welding was performed under different magnetic induction,after that the microstructures of the joints were analyzed to study the influence of the magnetic field on weld structure. Typical defects of the joints were studied,and the effects of arc swing and electrode position on weld formation were studied. Mechanical properties of the joints were tested. The results show that the applied magnetic field has the effect of fine grain and size of needle-type martensite in the weld was significantly decreased. The external magnetic field can effectively avoid the problem of poor sidewall fusion and achieve joints with good fusion sidewall. Electrode should be strictly placed in the center of the gap in order to obtain a uniform sidewall penetration. Welded joints had good mechanical properties,the average tensile strength of the joints reached 96% of that of the base metal.