The invention provides a method and a device for predicting deformation of a tunnel support structure excavated by a step method, which are applied to the technical field of tunnel excavation deformation prediction. The method comprises the steps of determining basic parameters; determining operation procedures required by tunnel excavation and time corresponding to each operation procedure; determining a first corresponding relation between the vertical surrounding rock pressure and time and a second corresponding relation between the horizontal surrounding rock pressure and time; calculatingthe horizontal deformation amount and the vertical deformation amount of the spraying layer structure in each construction cycle time, and drawing a curve of a tunnel deformation calculation value and time, wherein the total deformation after accumulation is an allowable deformation value of the structure; acquiring an actual measurement displacement value of a tunnel construction site, and drawing a curve of the tunnel deformation actual measurement value and time; and determining a construction adjustment measure based on comparison of the allowable deformation value, the curve of the tunnel deformation calculation value and time and the actual measurement result. By applying the embodiment of the invention, the deformation prediction in the tunnel excavation process by the step methodcan be realized, and the guiding effect of monitoring measurement on construction is effectively improved.