In this study, the 110601 working face gob-side entry retaining project in Nazuo Coal Mine, Fuyuan County, Yunnan Province, China, was taken as the background to explore the strength and economic benefit of the self-developed WPKJ-4 admixture used for filling material in gob-side entry retaining. The uniaxial compression test reveals that after the addition of a small amount of WPKJ-4 admixture, the strength of the filling material was significantly elevated, even higher than that of the original rock. Then, the relationship between the roadway-side backfill pillar width and the material strength was analyzed by a comprehensive approach combining the estimation method and the separated rock block method. The results obtained by the estimation method were safer and more reasonable. On this basis, the sensitivity of factors affecting the filling material strength was deeply analyzed. The sensitivity of each factor from high to low is: hc > γ > n > b2 > φ0 > λ > b1 > c0 = H = k > b3 > px, where hc (mining height), γ (unit weight of overlying strata, and n (multiple of roof pressure) are the main factors influencing the filling material strength of gob-side entry retaining in the 110601 working face. Finally, the direct economic benefit brought by adding a small amount (1.0%) of WPKJ-4 admixture to the filling pillar of the gob-side entry retaining in the 110601 coal mining working face was calculated to be 2.78 million yuan. It has been verified that the WPKJ-4 admixture is effective as a filling material for gob-side entry retaining.