Terahertz(THz) gyrotron is one of the specially developed high power THz radiation resources, and have important applications in biology, physic and faraway exploration. The cavity structure and physical mechanism of a 0.3 THz low voltage gyrotron for TE06 mode is analyzed. The start-oscillation current is calculated to suppress mode competition. The beam-wave interaction parameters are chosen in Particle-In-Cell(PIC) simulation. TE06 mode appearance, the output power of 152 W and output frequency of 299.5 GHz are obtained by beam-wave interaction simulation at 10 kV and 300 mA. Finally, output power in different magnetic fields and work currents is analyzed, which can provide important reference for the development of terahertz gyrotron.