Abstract: To meet the requirements of broadband harmonic mixers for terahertz instruments, passive imaging, high-speed communication systems, and other systems, a terahertz broadband subharmonic mixer with a working frequency covering dominant mode (220 GHz~330 GHz) of the standard rectangular waveguide WR-3.4 was developed using the planar Schottky diode. The subharmonic mixer circuit used a 25 um thick gallium arsenide (GaAs) as the supporting substrate. In this paper, the structure of the Schottky diode has been improved by using "T-shaped" structure to achieve a low parasitic anode junction, which can reduce the parasitic characteristics of the Schottky diodes, and improve the frequency performance. The monolithic integration technology was used to achieve a micrometer level alignment between the Schottky diode and the passive circuit, ensuring the implementation of broadband subharmonic mixers. Experimental results show that the single sideband (SSB) conversion loss of the 220 GHz~330 GHz sub-harmonic mixer is less than 12 dB in the range of 220 GHz~330 GHz, the intermediate frequency working bandwidth is up to 35 GHz (the SSB conversion loss is less than 12 dB), and the compression point of 1 dB is about -4 dBm. In addition, the consistency of test results from multiple sub-harmonic mixer samples confirms the potential for engineering application.