The embodiments of the present invention disclose a message transmission method, user equipment, a base station, and a communications system. The message transmission method in embodiments of the present invention includes: receiving, by first user equipment, information about a contention resource from a cellular network base station, where the contention resource is a transmission resource obtained in an autonomous contention manner; and contending, by the first user equipment, for a transmission resource in a time window corresponding to the contention resource, and if the transmission resource is obtained, transmitting a message to second user equipment by using the obtained transmission resource.