A speed reducer and a robot are disclosed. The speed reducer includes a rigid wheel, a flexible wheel and a flexible bearing; the rigid wheel is provided with a first fitting position and an inner wheel tooth set; the outer peripheral surface of a peripheral wall of the flexible wheel is provided with an outer wheel tooth set; in the axial direction of the rigid wheel, the tooth top of the outer wheel tooth set is provided with a first length, and the tooth root of the outer wheel tooth set is provided with a second length; the tooth top of the inner wheel tooth set is provided with a third length; the flexible bearing is provided with a fourth length; the second length and the third length are both greater than the first length, and the fourth length is greater than the second length.