Determining the gas stored in shale is critical for the evaluation of gas in place and the selection of resource potential areas. The application of USBM (US Bureau of Mines) desorption experiment is the most common method, which adopts the buoyancy principle for measuring shale gas content. Shale reservoirs are tight with strong heterogeneity and diverse curve patterns of measured desorption data and the square root of time. Due to the limitation of testing principles and equipment, the precision and accuracy of USBM measurement can not satisfy the demand for shale gas exploration and development. Here we report the development of a new method to measure the amount of desorbed gas in shales with high resolution. The device adopts the capillary pressure principle instead of buoyancy and mainly includes a sealed tank, gas collecting cylinder, and experiment box. Lost gas is an important part of shale gas content，which is unavoidable in the process of drilling coring. Lost gas content can be estimated by field desorption results. This practical tool provides some insights into the assessment of shale heterogeneities and the prediction of sweet spots.