The invention relates to a monitoring method, device and system of horizontal deformation of a bracket. A curve differential equation of the horizontal deformation of a stand column is established according to the bending stiffness EI of the to-be-monitored stand column of the bracket; the stand column of the bracket is formed by connecting a plurality of sections end to end; a mapping relationship between a horizontal deformation value of the stand column and an inclination angle of the stand column in a single section in the vertical bridge direction or the horizontal bridge direction; at least three inclination angle data in all of the single section ranges are collected; the at least three inclination angle data in the single section ranges are input into a complex Simpson numerical integration model; and the horizontal deformation value of the stand column of the bracket is generated to realize the monitoring of the horizontal deformation of the bracket. The method can accuratelymonitor the horizontal deformation of the bracket and is high in monitoring precision; and powerful guarantee is provided for subsequent safe production.