In accordance with the character of diffusing Ga and Al in SiO2/Si system and in the use of magnetic-control device for controlling accurately the doping quantity of Ga, the double-impurity doping of both Ga and Al is completed consecutively in the same high-temperature diffusion furnace. It is shown that the doping is with good uniformity，repeatability and continuity．A uniform distribution of impurity concentration can be achieved．Since the covalent radiuses of the atoms，Ga and Al，are close to that of Si，and the temperature is slowly dropped after the high temperature, the defects of the lattice is obviously reduced．The lifetime of minority carrier is significantly increases and the voltage drop is reduced．In this paper, the mechanism of enhancing the performances of thyristors by Ga-A1 doping is investigated．Our result shows that the double-impurity doping technique is conducive to raising the level of breakdown voltage and surge capacity，and improving the characteristics of current，triggering and dynamics significantly．The double-impurity doping technique is better than other known techniques of doping．