The UWB (Ultra Wide Band) technology can play an effective role in FCS (Fieldbus Control System). But the channel acquisition time of UWB is high, and a lot of packets are short in FCS. So the efficiency of FCS based on UWB will significantly reduce. In this paper, a CSMA/CA protocol with D-UF (Dynamic Length Mechanism for Unite Frame) is proposed. The characteristics of UWB and size of data packets in FCS are taken into account in the CSMA/CA protocol with D-UF. First, the characteristics of UWB and FCS are discussed, and the slot utilization of CSMA/CA protocol is analyzed in different FCS. Finally, the packet assembly policy is designed. At the same time, the performance of the CSMA/CA protocol with D-UF is evaluated in FCS based on UWB by modeling and simulation. Simulation results show that the CSMA/CA protocol with D-UF is effective to increase slot utilization, and improve throughput, and reduce average delay in the FCS based on UWB. It is useful to engineer for designing the FCS based on UWB.