A design method of a dumper speed reducer comprises the steps of determining the working environment and related information of a whole dumper, determining the working environment and design requirements of the speed reducer, determining the mounting requirements of the speed reducer and the dumper, determining the input load of the speed reducer according to the requirements of a motor, determining the transmission mode of the speed reducer, conducting model selection calculation on a gear and a bearing of the speed reducer, conducting finite element analysis on a planet carrier of the speedreducer, and conducting model selection on the lubrication mode, the sealing mode and the fastening mode of the speed reducer. The actual use requirements are met by theoretical calculation.