The Network Capacity Issues on Designing Routing Protocol of Vehicular Adhoc Network According to the characteristics of fast nodes, frequently varying topological structures and instable wireless channel quality of Vehicular Ad-hoc networks (VANET), the paper proposed a scheme which the factors such as business demands, network abilities and VANET feathers should be considered comprehensively on designing the routing protocol of VANET. Analyzing variant demands of transmission rate on loading business which include low rate pilot channel and high rate traffic channel, upper and lower limit of network capacity on choosing routing protocol is explored. Experimental results show that the practical limit of the node throughput increases with radio waves attenuation coefficient and node bandwidth.