The invention provides a horizontal well chemical flooding mining method. The method includes the steps: deploying a single horizontal well injection-production well pattern in an oil layer, deploying an injection well in a higher-permeability region and deploying an oil production well in a lower-permeability region; alternately injecting polymer-surfactant flooding solution into the injection well at the speed of 0.12-0.2PV/a; performing subsequent water flooding until water content reaches 98%; finishing production when the comprehensive water content reaches 98%. The planar pattern direction of a horizontal well is parallel to a source direction, a bottom injection and top production mode is adopted in the same oil layer, the length of a horizontal section of the horizontal well is 50-150m, and the distance between adjacent horizontal wells is 100-200m. According to the mining method, the recovery ratio of a water-flooding reservoir is greatly increased, the development effect of the oil layer is improved, and a new breakthrough is achieved in terms of chemical flooding technology.