The present invention relates to a gas density sensor, comprising a casing, a first crystal resonator, a second crystal resonator, a crystal sealing cavity, a first resonant circuit, a second resonantcircuit, and a signal processing circuit; wherein the first crystal resonator is exposed to gas to be tested, the second crystal resonance is encapsulated in the crystal sealing cavity, inside the crystal sealing cavity is reference gas or vacuum, and the signal processing circuit test difference between resonant frequencies of the two crystal resonators through the first resonant circuit and thesecond resonant circuit, transmits the difference signal to the terminal display device and determines density of the gas to be tested through relationship between the resonance frequency differencesignal and the density of the gas to be tested. The gas density sensor has small volume, low cost, a compact structure, high measurement precision and long service life.