The variation of equilibrium in the tokamak plasma is derived by solving the integral form of the linearized Grad-Shafranov (GS) equation. The state space description of the evolution of the plasma equilibrium is obtained by combining the solutions to the linearized GS equation with the circuit equation of flux conservation. Vertical displacements of plasmas are analysed using the equilibrium of the compact ITER, and the results are compared with a rigid model of plasma evolution. In the case of the compact ITER, the linear growth rates derived by the linearized GS equation are larger than those derived by the rigid model. The quantitative characterization of the non-rigid motion of a plasma is discussed. To assess the validity of the simulation method, a minor disruption observed in a JT-60U experiment is simulated and good agreement between the simulation and the experiment is found.