Hidden/exposed nodes are key factor which affects the reliability of broadcasting information and the MAC Contention Window is another factor which affects the throughput for single-hop broadcasting protocols in VANET. Firstly, this paper provides a theoretical analytic model to analyze the relationship among reliability, throughput, node density and Contention Window. Next, based on multi-object optimization theory, an adaptive reliability single-hop broadcast protocol (ARSB) is proposed. Simulation results show that ARSB not only enhances broadcast reliability and satisfies the broadcasting information real-time needs, but also optimizes broadcast throughput by adapting Contention Window of broadcasting nodes.