The Meissner effect is a special property of superconductors. In this paper, based on the characteristics of the Meissner effect, a superconducting phase transition model of a superconductor under a changing magnetic field is proposed. A thermodynamic analysis was performed on the model to calculate both the changes in the thermodynamic parameters of the superconductor during the phase transition and the changes in the overall electromagnetic quantity of the model. For Model 1 in the present study, the thermodynamic electromagnetic energy of the superconductor was not conserved due to the phase change of the superconductor under a changing magnetic field.