The present invention discloses a random access method, a mobile terminal, and a mobile communications system, and relates to the communications field. The method includes: obtaining a dynamic probability class (N) and an access service class (ASC) of a current service that are in a system information block; calculating an access probability of the current service according to the N and the ASC of the current service; successively comparing random numbers corresponding to available access timeslots with the access probability of the current service until a random number corresponding to an available access timeslot is less than or equal to the access probability of the current service; and sending an access preamble to a base station in the available access timeslot. Embodiments of the present invention apply to establishment of a random access process for a mobile terminal.