A large portion of slickwater is trapped in gas shale formations after the hydraulic fracking process. To address the effect of slickwater retention on methane desorption, the adsorption and desorption isotherms for methane before and after slickwater treatment were measured based on the volumetric method using shale samples from the Longmaxi formation, China. We found the adsorption and desorption data for the methane before and after treatment fit well with the Freundlich model. We compared the effect of slickwater volume on desorption isotherms, desorption efficiency, adsorbed methane content and cumulative gas production. Slickwater treatment significantly reduces methane adsorption. The Freundlich constant k reduction decreases nearly linearly with the slickwater volume, and constant n increases nearly linearly first and then becomes slower with slickwater volume. Test results also reflected that slickwater treatment could improve methane desorption efficiency. The curve of desorption efficiency after treatment is significantly lower than before. Under 30 MPa pressure, the desorption efficiency increases by 50.93% when slickwater volume increases to 10.40 ml from 2.02 ml. A closed loop exists between two curves of adsorbed gas content before and after treatment, and its area increases with slickwater volume. Slickwater treatment improves cumulative gas production. The cumulative production of methane before treatment is smaller than after. 8.16 ml of slickwater is the best option for 130 g of shale powders to obtain the most cumulative gas production in the lab-scale condition. These results can be used to optimize the slickwater volume during hydraulic fracturing.