The invention discloses a length determining method for an anchor rod for roadway supporting. The length determining method includes the steps that according to the shape of a cross section of a roadway, a circumcircle of a contour line of the roadway is made; then an expansion concentric circle of the circumcircle is made according to the development depth of a surrounding-rock plastic zone, and parameters of the reasonable length of the anchor rod are obtained in the mode that the distance from the surface of the roadway to the expansion concentric circle of the plastic zone, the length that the anchor rod is anchored into hard-and-stable surrounding rock and the exposed length of the anchor rod are superposed. According to the length determining method, the process that the reasonable length of the anchor rod is determined is simple, operation is convenient, the accuracy is high, materials of a rod body of the anchor rod are saved, waste is avoided, the construction speed of drilling is increased, the labor strength of workers is reduced, the surrounding-rock control efficiency of the anchor rod is improved, and the favorable condition is provided for mine safety production.