A harmonic reducer with an oil guiding ring, wherein an oil guiding ring is disposed in an accommodating space of a flexible external gear, and the oil guiding ring is sleeved on a coupling member and located between a first housing and a flexible bearing. Therefore, after lubricant oil is injected through the oil filling hole of the first housing, the lubricant oil enters an annular oil storage groove through the oil inlet holes of the oil guiding ring. When the wave generator rotates, the lubricant oil in the annular oil storage groove is guided to the flexible bearing of the wave generator by the annular wall surface to lubricate the flexible bearing. Therefore, on the basis of effective lubrication, the harmonic reducer can avoid the foreign body impurity caused by insufficient lubrication, and at the same time, does not have to change the original design.