Method for selecting random access channel by user terminal in radio mobile communication system

In the invention, the radio communication system where the user end locates at is a multi carrier system, in which each cell is allocated to at least one carrier, and the uplink synchronizing channel associated with the random access channel exists on the carrier of at least one said cell; measuring the signals quality of uplink synchronizing channel on each carrier, and according to the channel quality measuring result, setting the quality level of the uplink synchronizing channel on each carrier; then, according to the quality level of the uplink synchronizing channel on each carrier, setting the dynamical keeping level of its associated random access channel, and informing the dynamical keeping level of its associated random access channel to said user end; finally, according to the dynamical keeping level of said random access channel, user end makes selection from available random access channels.