Abstract: The isotope monitoring of desorption gas from shale cores was carried out at the drill site. Gas is lost during core recovery and thus the isotope variation for the whole process of gas release from shale cores cannot be obtained. A high pressure methane saturation-desorption device was developed and connected to the chromatographic-isotope mass spectrometry system to monitor the carbon isotope change of released CH₄ on-line. The isotopic curves of releaed methane were initially stable, and then tended to become lighter and finally heavier, which was related to the phase transformation and the mixing ratio of free gas and adsorbed gas. Isotopes have potential applications in monitoring shale gas development.