Speed reducer capable of collecting oil in two directions and electric vehicle

The speed reducer comprises an input wheel, a middle wheel set, an output wheel and an oil collecting groove, the input wheel is used for driving the output wheel to rotate forwards or backwards through the middle wheel set, the rotation direction of the middle wheel set is opposite to that of the output wheel, and the oil collecting groove comprises a radial notch and a circumferential notch. Wherein the radial notches face away from the middle wheel set in the radial direction of the middle wheel set. The circumferential notches face the output wheel in the circumferential direction of the middle wheel set. And the radial notches are communicated with the circumferential notches. The radial notches and the circumferential notches are formed in the oil collecting groove, so that the oil collecting groove can collect lubricating oil stirred by the gear train during forward rotation or reverse rotation when the input wheel drives the output wheel to rotate forwards or reversely through the middle wheel set, th