Avionics Full Duplex Switched Ethernet (AFDX) standardized as ARINC 664 is an upgrade from Ethernet for avionics demand.AFDX adopts the mechanisms such as Virtual Link (VL) and traffic policing for the determinacy of communication task.Network calculus approach is a basic tool to compute the upper bound of end-to-end delay for VL in AFDX network which give AFDX a useful method to research the real-time performance.However the network calculus approach used a simple flow model of VL.Based on that model the result got by network calculus approach is pessimistic.An accurate flow model of VL is adopted in the network calculus approach in this paper and it helps the network calculus approach to get a tighter upper bound of delay for VL.