The utility model provides a tension wheel assembly of a single-cylinder diesel engine, which is mainly composed of a tension wheel, a wheel shaft, a bearing and a tension regulating plate, wherein the tension wheel is arranged on the wheel shaft and is limited by a lug boss at one end of the wheel shaft. A singe flange bearing is arranged on the wheel shaft and is arranged in the tension wheel. An opening of the bearing faces the lug boss end of the wheel shaft, and the other end of the wheel shaft is provided with the tension regulating plate which is fixed by a lock plate and a nut. The lug boss end of the wheel shaft is provided with a screw cap type oil cup, and an oil passage communicated with an oil outlet of the screw cap type oil cup is arranged on the wheel shaft. Both end openings of the oil passage are positioned in the opening positions of the single flange bearing. In order to improve the working reliability of the tension wheel assembly, a belt groove on the tension wheel is an A-shaped belt groove. Because the screw cap type oil cup is additionally arranged and the single flange bearing is used, the utility model has the advantages of reasonable structure, easy fabrication and convenient maintenance, the working reliability of the tension wheel assembly is improved and the service life is extended.