The invention discloses an avalanche diode high-frequency parameter low-temperature testing system. The avalanche diode high-frequency parameter low-temperature testing system includes a testing chamber. A lifting bracket is arranged at the bottom in the testing chamber, and a medium cooling table is arranged at the top end of the lifting bracket. A temperature sensor and an oscillator for loadingan avalanche diode to be tested are arranged on the medium cooling table, and a circulator is arranged above the oscillator. A first waveguide element, a second waveguide element, a first electricalconnector and a second electrical connector are separately arranged on the side walls of the testing chamber. An injection lock signal source, an avalanche tube reverse bias power supply, a power meter, a high-frequency test loop, a harmonic mixing detection unit, a temperature sensor power supply, a vacuum pump, a liquid nitrogen tank, a program-controlled valve, a flow meter, a liquid nitrogen recovery device, a microcontroller and a display screen are arranged outside the testing chamber. The avalanche diode high-frequency parameter low-temperature testing system can effectively overcome the existing technical defects that during a low-temperature test of an avalanche diode, the output efficiency of a millimeter wave avalanche diode is affected to be reduced due to increased transmission loss caused by condensation and frosting of a transmission circuit, and the accuracy of device testing is ensured.