The invention discloses a gas electrostatic density measuring instrument and a method. The gas electrostatic density measuring instrument comprises a charge induction sensor, a vibration capacitance sensor, a signal conditioning module and a control module. The main measuring principle is that through measuring a potential difference generated by charges in the gas on the outer wall and the inner wall of the charge induction sensor, the charge density of the charged gas is calculated. The charge density of a thunderstorm gas can be measured via the gas electrostatic density measuring instrument; due to the design of a dual-cylinder structure, influences of an external gas on measurement can be effectively avoided; the circuit is simple; stability is good; the measurement precision is high; the application prospect is good; and popularization is worthwhile.