In this paper, we derive a rigorous expression of the nuclear gradient using the the QED Hamiltonian without making unnecessary approximations. Under the adiabatic-Fock representation, and requiring the total energy conservation of this MQC system, we derived the rigorous nuclear gradient, and it is intuitively connected to those approximate gradients under the JC approximation. This rigorous expression has additional terms that go beyond the expression of the JC Hamiltonian. We have numerically demonstrated the importance of these terms in the MQC simulations of polariton dynamics.