The real-time performance of switched Ethernet is a hot research field. However, there isn't an effective modeling to describe it until now. Through analyzing the structure of classical switch, a switch model by network calculus is proposed, in which the data flow is divided into real-time flow and non-real-time flow and the arrival curve, service curve, the max delay and max storage needed are got. The max delay provides theory direction for transmitting real-time data in switched Ethernet and the max storage provides the theory reference for constructing switch in real-time industrial Ethernet.