The invention discloses a trunked communication system adopting time division multiple access, a device and a channel allocation method. The channel allocation method comprises the steps that a service request from a terminal is received, and the service type of the requested service is analyzed; according to the obtained service type through analysis, when an idle and available channel exists in a started channel machine, the idle and available channel of the started channel machine is allocated to the service, a frequency point corresponding to the allocated channel is sent to the terminal, and therefore the terminal is switched to the allocated channel. According to the technical scheme, the idle and available channel of the started channel machine is allocated to the new service preferentially, so that the starting frequency of the channel machine is lowered, waste of channel resources is avoided, electric energy is saved, the access rate of base station service is greatly improved, the service life of a base station is prolonged, and the service access capacity of the base station is improved.