In this paper some properties of n+k (k≥4) phase multisystems are discussed and thesymbolism depicting them is proposed systematically. Upon this, the combination principle forclosed nets of n+k phase multisystems is preseted and elucidated. The combination principlestates: Any closed net of one n+k (k≥4) phase multisystem must be a combination of two ormore distinct n+3 order submultisystem closed nets belonging to the given n+k multisystem,if it is not one of the n+3 order submultisystem closed net itself. The presentation of thecombination principle provides both the theoretical basis and the practical way for investigationand solution of the topological configurations of the phase diagrams of any n+k phase multisystems. In this paper some properties of n + k (k≥4) phase multisystems are discussed and the symbolism depicting them is proposed systematically. Upon this, the combination principle forclosed nets of n + k phase multisystems is preseted and elucidated. The combination principlestates: Any closed net of one n + k (k≥4) phase multisystem must be a combination of two ormore distinct n + 3 order submultisystem closed nets belonging to the given n + k multisystem, if it is not one of the n + 3 order submultisystem closed net itself. The presentation of thecombination principle provides both the theoretical basis and the practical way for investigation and solution of the topological configurations of the phase diagrams of any n + k phase multisystems.