The invention provides governing mechanism of a guide plate of a horizontal roll mill, comprising a rotating shaft, an eccentric wheel, a rocker arm and a dial. An end of the rotating shaft passes through an end cover of the horizontal roll mill while the external end of the rotating shaft is arranged in the horizontal roll mill and the inner end thereof is positioned outside the horizontal roll mill. The internal end of the rotating shaft is provided with the eccentric wheel which is in contact with the guide plate while the contact point of the eccentric wheel and the guide plate deviates from the axial axis and the radial axis of the guide plate. The external end of the rotating shaft is provided with a rocker arm with a handle. The inner side of the rocker arm is provided with the dial. The dial is provided with a plurality of regulating ports which are distributed in arc-shaped manner. The rocker arm is provided with pins corresponding to the regulating ports. An axle sleeve is sheathed on the rotating shaft positioned in the horizontal roll mill. A bearing is arranged between the axle sleeve and the rotating shaft. Two ends of the axle sleeve are sealed by a flange and a sealing ring. The governing mechanism has the advantages of reasonable and compact structure, convenient and flexible operation, high working reliability and long service life, can realize online regulation of the guide plate of the horizontal roll mill, effectively improve the working efficiency and realize optimization of the working parameters of the horizontal roll mill.