The invention belongs to the technical field of oil and gas field exploration and development and particularly relates to a method for implementing second-time well cementing in a horizontal well section. The method comprises the following steps that a start position and a termination position of the well section to be treated are determined; the well section to be treated is perforated; the inner wall of a sleeve is scraped; a cement retainer is set on the sleeve and released; a sleeve gate is opened, and clean water is pumped into a drill rod; after backwater is continuous and stable, cement paste is injected; an upright column is lifted up, after lifting, a reverse circulation well washing pipeline is better connected, the sleeve gate is opened, large discharge capacity of clean water conducts reverse circulation for at least 2 weeks, a well is shutdown for cement waiting; and a working pipe column of second-time well cementing in the well is pulled out, and a drilling and grinding pipe column is placed to drill out retentate in a shaft; and pressure test is conducted on the shaft. Therefore, according to the method, fluid injected in a cement ring gap can be effectively displaced, the fluid is replaced with the qualified cement paste, under-pressure cement waiting is realized, and the prior art is completely replaced to implement effective second-time well cementing in the horizontal well section.