Since network users have different socio-economic characteristics and different levels of income，this paper proposes a generalized system optimal model in traffic networks with fixed demand and then explicitly derives its first-order optimality conditions． The proposed model can deal withany continuously distributed value of time ( VOT) or finite discrete VOT． The relationships among the generalized system optimal model，standard system optimal model and multi-class cost-based system optimal model are investigated． It is found that，for the fixed demand traffic networks with homogeneous users，the generalized system optimal model degenerates into a standard system optimal model，and for traffic networks with multi-class users，the generalized system optimal model and multi-classes cost-based system optimal model are equivalent．