Aiming at the problem of borehole collapse of shale gas extended reach horizontal wells, the mechanism of water swelling of shale minerals is obtained by analyzing the micro-structure of shale. A new shale inhibitor, an alkyl glucoside quaternary ammonium salt (QASAPG), was developed and its mechanism of action on shale was investigated. Through multiple experimental methods such as cuttings roll recovery rate, relative inhibitory rate, shale swelling, water activity measurement, and particle size distribution, the inhibition performance of QASAPG was evaluated. The corresponding results indicated that under the effect of the QASAPG, the first recovery rate and the relative recovery rate of cuttings were greater than 94.7% and 99% respectively; the reduction rate of the height of shale swelling reached 67.3%. The QASAPG aqueous solution activity is only 0.67; QASAPG can significantly reduce the content of submicron particles in drilling fluid; and QASAPG exhibited excellent inhibition performance. Experiments results showed that the excellent inhibition performance of QASAPG was mainly due to it being embedded into the layered structure, straining the crystal layers, forming films by electrostatic effects and hydroxyl adsorption, reducing the water activity and forming a sealing layer. However, the inhibition performance of alkyl glucoside was mainly from forming films and reducing the activity of water under high concentrations. Through the analysis of the inhibition mechanisms, it can be well explained that the inhibition performance of QASAPG is far superior to that of alkyl glucoside and this has also been proved by field application. With the special properties of the film effect, QASAPG may make a contribution to the drilling industry and even in hydraulic fracturing.