A mechanical-double ring-hydraulic composite transmission mechanism includes an input member, a convergence mechanism, a hydraulic transmission mechanism, a double ring mechanism, an output member, a clutch assembly, and a brake assembly. The clutch assembly connects a left planetary gear train to a right planetary gear train. The clutch assembly connects the input member to the hydraulic transmission mechanism, the double ring mechanism, the left planetary gear train, and the right planetary gear train. The clutch assembly connects the hydraulic transmission mechanism to the right planetary gear train. The clutch assembly connects the double ring mechanism to the left planetary gear train. The clutch assembly connects the left planetary gear train and the right planetary gear train to the output member. A continuous transmission ratio between the input member and the output member is provided by adjusting a displacement ratio of the hydraulic transmission mechanism.