In China, one or several ultrathin soft coal sublayers are widely developed in the coal seam. Therefore, a method of using hydraulic flushing in soft sublayer to enhance the gas extraction in these particular coal seams is developed in this work. We first established a new fully coupled gas extraction model by combing gas diffusion, gas flow, and a permeability model that considers the effect of stress change and plastic failure. By adopting this model, the gas extraction enhancement mechanism and its main influence factors were studied using the numerical simulation method based on the engineering and geological background in the Yangquan No.5 coalmine. Thereafter, a hydraulic flushing equipment, which could move freely in the underground coalmine, was developed to apply the hydraulic flushing method in soft coal sublayers used in the 8402 working face. Its gas extraction effect was systematically investigated. Our simulation results match well with the field results, suggesting that our model is feasible. Meanwhile, after adopting this method, the gas extraction condition in this coalmine improves significantly. The borehole number decreases by 66.7%, while the gas extraction rate and gas extraction concentration increase by 1.33 times and 3 times, respectively. Moreover, during the coal mining process, the gas adsorption index of drilling cuttings, the quantity of drilling cuttings, and the CH4 concentration also decrease dramatically.