In order to develop compact, reliable and high power terahertz radiation sources, multi-mode second-harmonic operation in a terahertz gyrotron was investigated theoretically and experimentally. The theoretical calculation results show that there are five second-harmonic modes which operate at single-mode oscillation state in the gyrotron by adjusting the design parameters. Experimental results demonstrated that there are four second-harmonic modes (TE16, TE64, TE46 and TE26 ) in the gyrotron operating with a 1.5A/41kV electron beam. Output powers of 1.5, 2.1, 3.0 and 1.4 kW can be generated at the corresponding frequencies of 0.391, 0.404, 0.416 and 0.423 THz, respectively.