We derive a new perturbative quantum master equation for the reduced density matrix of a system interacting with an environment (with a dense spectrum of energy levels). The total system energy (system plus environment) is constant and finite. This equation takes into account the finite energy effects of the environment due to the total energy conservation. This equation is more general than the common perturbative equations used for describing a system in interaction with an environment (like the Redfield equation or the Cohen-Tannoudji one) because these last equations can be deduced from it in the limit of an infinitely large environment. We apply numerically this equation to the spin-GORM model. This model represents the interaction of a two-level system with an environment described by random matrices. We compare our equation with the exact von Neumann equation of the total system and show its superiority compared to the Redfield equation (in the Markovian and non-Markovian cases).