In the THz frequency band, the low quality factor of passive device capacitance inductance, the influence of parasitic circuit parameters and cut-off frequency of Metal-Oxide- Semiconductor Field-Effect Transistor(MOSFET) make it difficult for THz oscillator circuit to achieve high power output. A 300 GHz tuning oscillator is proposed to achieve high power output. Firstly, the structure of cross coupled Push-Push oscillator is adopted to realize the second harmonic signal output of 300 GHz. Through the method of power synthesis, the proposed oscillator exports the second harmonic 300 GHz signal, which increases the output power of the oscillator and breaks through the cut-off frequency of MOSFET. The gate interconnection inductance is also adopted to increase the output power. Secondly, instead of tuning with a variable capacitor on the chip, the Push-Push oscillator modulates the substrate voltage of MOSFET to adjust the parasitic gate capacitors of MOSFET to achieve frequency tuning, further increasing the output power. The proposed push-push oscillator is fabricated in TSMC 40 nm CMOS process. By extracting the second harmonic signal and using the substrate voltage tuning technology, the proposed coupled Push-Push oscillator has a peak output power of -2.2 dBm at 309.0 GHz and achieves 303.5-315 GHz tuning range. The phase noise of the oscillator is -79.5 dBc/Hz at 1 MHz offset and the power consumption is 28.6 mW.