A 440 GHz series frequency doubler based on planar Schottky varactor diode is studied. Under the driving of the fundamental power of 20 mW, the simulation result shows that the 400-480 GHz conversion loss is less than 10 dB, the output power is greater than 2 mW, and the relative bandwidth is 18%. The whole frequency multiplier is manufactured on a 30 μm thick quartz substrate, which has no mechanical tuning structure. It is simple and convenient to manufacture.