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. <EOS>