The present invention provides a gas measuring method based on photothermal effect in hollow-core optical fiber comprising: filling a target gas into the core of a hollow-core optical fiber; coupling a probe light and a periodically modulated pump light into the hollow-core optical fiber; absorbing the pump light by the target gas resulting in the periodic modulation of the phase of the probe light; demodulating the phase modulation information of the probe light to obtain the concentration of the target gas, wherein the pump laser is wavelength and/or amplitude modulated. In the present invention, two lasers including a pump laser and a probe laser are used for the measurement, this approach is simple and practical. Also, the use of the hollow-core optical fiber with extremely-small core area greatly increases the optical power density, thus enhances the strength of the detected photothermal signal; this method allows ppb level gas measurement with high selectivity, and is universally suitable for the detection of gases with absorption in near-infrared.