Mixer is widely used in communication receiving system. With the development of the microwave communication system towards high frequencies, the millimeter wave mixer of high frequencies is becoming the millimeter wave device which is the key researched. This paper presents a novel structure design of millimeter broadband mixer. The LO signal input from the rectangular waveguide to suspended microstrip line and the RF signal from a rectangular waveguide to a fin line. The suspended microstrip lines and finline with a pair of diodes rested between them constitute a balanced mixing circuit. Through the simulation of the millimeterwave broadband mixer, it owns a 75~105 GHz operating frequency, and its conversion loss is better than 12 dBm. The isolation among the three ports is also excellent. In addition, the mixer’s structure is very simple and easy processed.