A broad-tunable free electron terahertz radiation source is proposed. In this source, a train of electron bunches with tunable bunching frequency is produced by a photocathode based DC-gun under excitation of a train of tunable laser pulses. These electron bunches are then applied to excite an over-mode waveguide, in which diverse guided modes are coupled into radiation with frequency determined by the bunching frequency. By this means, the tunable radiation with frequency extending from 0.1 THz to 1.2 THz can be obtained from one single structure model. In addition, compared with other sources, the proposed source is compact and easily achievable.