The invention discloses a steel rail online heat treatment method and a restraining device thereof in the technical field of metal heat treatment. The method comprises the steps that the temperature of a rolled steel rail is controlled at 700-800 DEG C; the steel rail is sent to the restraining device, an accelerating cooling medium is applied to the center of a steel rail head tread surface, thetwo sides of a rail head and a rail bottom center by the restraining device, and the cooling speed is 1-5 DEG C per second; the restraining device is used to apply a force to the steel rail so that the steel rail can realize S-shaped bending deformation on a horizontal surface after passing through the restraining device; and when the temperature of the steel rail is lowered to 450 to 500 DEG C, the steel rail is sent out of the restraining device. S-shaped bending deformation of the steel rail is realized to generate crystal defects such as dislocations, so that promotes pearlite nucleation is promoted, pearlite lamellar spacing is reduced, and rail strength and toughness are comprehensively improved; and the accelerated cooling medium is applied to the steel rail head and the steel railbottom to rapidly reduce the temperature of the steel rail head and the steel rail bottom, so that the residual stress of a steel rail waist is reduced and the overall performance of the steel rail isimproved.