The invention discloses an attenuated method for fowl infectious bronchitis viruses. The method comprises the following steps: the toxic strain YX10 is inoculated on SPF chick embryos, 103.5-4EID50/0.2mL of the toxic strain is inoculated on every embryo; after cultured for certain time, the chick embryos are placed at the temperature of minus 20 DEG C for half an hour; allantoic fluids are collected in sterile conditions; the passage of the next generation is carried out; the culture temperature of the first generation to the tenth generation is 37 DEG C and the culture time is 48 h; the culture temperature of the eleventh generation to the fourteenth generation is lowered by 1 degree per generation and the culture time is 48 h; from the fifteenth generation, the culture temperature is 32 DEG C, and the culture time is 32-48 h. The method can save the passage time effectively, and reduces the loss of immunogenicity. The method is beneficial to develop novel vaccines in time and effectively.