Abstract: Using energy balance equations in classical Statistical Energy Analysis(SEA), the energy balance mechanism of non-conservatively coupled systems is studied, and calculating formulae of equivalent internal loss factors for non-conservatively coupled systems are derived. At the same time, the methods of engineering calculating of equivalent internal loss factors are given. By taking the two non-conservatively coupled oscillators for example, the influences of coulping parameters on equivalent internal loss factors are studied. From the analytical results, it is shown that negative internal loss factors result from energy balance of non-conservatively coupled systems.