Abstract: Based on energy balance equations for two coupled systems in Statistical Energy Analysis (SEA),the posslbility of causing negative loss factors in measuring SEA parameters by using the equations is analyzed.The physical causes of formation of negative loss factors are theoretically investigated by vibration analysis of the two non-conservatively coupled oscillator sets From the analytical results;it is shown that the necessary condition of forming negative loss factors is that the coupling of the coupled systems is non-conservative and strong and the sufficient condition is that the excitation frequency of the external force is not equal to the natural frequencies of the two oscillators at the same time or the parameters of the two oscillators are much different.The absolute values of the negative loss factors increase with the increase of the coupling parameters.