A resistive gas sensor is provided. The resistive gas sensor includes a sensing circuit and a determination circuit. The sensing circuit senses a gas to generate a detection signal. The determination circuit performs a frequency-division operation on the detection signal by a frequency-division parameter to generate a frequency-division signal, counts a half of a period of the frequency-division signal to generate a half-period count value, and determines concentration of the gas according to the half-period count value. The determination circuit determines the frequency-division parameter according to the half-period count value.