The invention discloses a speed reducer of an oil pumping unit. The speed reducer of the oil pumping unit comprises a speed reducer body, a pipeline, a circulating pump and a filter. The speed reducer body comprises a shell formed by connecting a machine base and a machine cover in a sealed mode. The machine base comprises a side wall and a bottom wall provided with an oil outlet. Machine oil is contained in the machine base. An oil inlet is formed in the shell. The pipeline is communicated with the oil outlet and the oil inlet. The circulating pump is arranged on the pipeline and drives the machine oil in the pipeline to flow from the oil outlet to the oil inlet. The filter comprises an outer shell and a filter element, wherein the outer shell is provided with a first connector and a second connector, the filter element is arranged in a filter cavity defined by the outer shell, the filter is arranged on the pipeline, and the filter is connected with the pipeline through the first connector and the second connector. When the speed reducer of the oil pumping unit is used, machine oil can be filtered without stopping, and the economic benefit of oil pumping is increased.