An automatic speed reducing wheel, comprising: a wheel unit having a wheel seat, a wheel axle passing through the wheel seat, and a roller rotating about the wheel axle, the wheel seat comprising a through hole through which the wheel axle passes, the wheel axle comprising a non-circular section on one end thereof, the roller comprising a wheel frame whose outer periphery is combined with an outer wheel, the wheel frame comprising an axle bore passing through a center thereof, the wheel axle passing through the axle bore, the roller being configured to freely rotate about the wheel axle, a plurality of blockers disposed on one side of the wheel frame in adjacent to an edge thereof in a radiation shape arrangement, the plurality of blockers being configured to rotate along with the roller; and a speed reducing unit, characterized in that the speed reducing unit comprises a fixing seat and a sliding member, the fixing seat comprising a fixing bore and a sliding bore, the fixing bore mounted around the non-circular section of the wheel axle and fixed thereon, the fixing seat located on one side of the roller, the sliding member slidably disposed in the sliding bore, the sliding member comprising a sliding body, the sliding body comprising an elastic portion connected with a front end thereof, the elastic portion comprising a blocking portion connected with a front end thereof; when the roller moves downhill, the sliding bore slants downward by an inclination angle, and the sliding member slides along the sliding bore due to a gravity force, such that the blocking portion of the sliding member protrudes out of an outer side of the sliding bore to interrupt a rotation route of the blockers, whereby the blockers pass and hit the blocking portion one by one, such that a cooperation between the blocking portion and an elasticity of the elastic portion achieves a speed reducing effect. a wheel unit having a wheel seat, a wheel axle passing through the wheel seat, and a roller rotating about the wheel axle, the wheel seat comprising a through hole through which the wheel axle passes, the wheel axle comprising a non-circular section on one end thereof, the roller comprising a wheel frame whose outer periphery is combined with an outer wheel, the wheel frame comprising an axle bore passing through a center thereof, the wheel axle passing through the axle bore, the roller being configured to freely rotate about the wheel axle, a plurality of blockers disposed on one side of the wheel frame in adjacent to an edge thereof in a radiation shape arrangement, the plurality of blockers being configured to rotate along with the roller; and a speed reducing unit, characterized in that the speed reducing unit comprises a fixing seat and a sliding member, the fixing seat comprising a fixing bore and a sliding bore, the fixing bore mounted around the non-circular section of the wheel axle and fixed thereon, the fixing seat located on one side of the roller, the sliding member slidably disposed in the sliding bore, the sliding member comprising a sliding body, the sliding body comprising an elastic portion connected with a front end thereof, the elastic portion comprising a blocking portion connected with a front end thereof; when the roller moves downhill, the sliding bore slants downward by an inclination angle, and the sliding member slides along the sliding bore due to a gravity force, such that the blocking portion of the sliding member protrudes out of an outer side of the sliding bore to interrupt a rotation route of the blockers, whereby the blockers pass and hit the blocking portion one by one, such that a cooperation between the blocking portion and an elasticity of the elastic portion achieves a speed reducing effect.