Discrete event simulation is increasingly finding more exten-sive uses in the analysis and design of complex systems.In simulation studies,simulation modelling is a very sophisticated and insurmountable effort.Studies on simulation modelling formalism can enhance the recog- nition of discrete event system and improvement of simulation modelling process,and construct a more valid simuation model.The effect of formal-ism in simulation modelling is discussed in the paper.A number of efforts in simulation modelling formalism are reviewed.Formal system specifica-tion based on system theory and the discrete event simulation formalism are presented.