The invention provides a composite molecular sieve and a hydrodesulfurization catalyst prepared with the composite molecular sieve as a carrier. According to the composite molecular sieve and the hydrodesulfurization catalyst, the Beta-FDU-12 composite molecular sieve is of a bimodal porous structure, namely a Beta micropore and an FDU-12 mesoporous, the superficial area of the composite molecular sieve is 800-1000m<2>.g<-1>, the pore diameter is 14-25 nm, and the pore volume is 0.4-0.9 cm<3>.g<-1>.The hydrodesulfurization catalyst prepared with the Beta-FDU-12 composite molecular sieve as the carrier is outstanding in performance, the yield of FCC diesel oil can be higher than 99%, the denitrification percent can be higher than 98%, and the sulphur content is lower than 10 ppm.