The soil around the first mining area of a company was taken as the research object, aiming at the engineering problems of soft and weak bearing capacity of Quaternary surface soil in coastal karst area. In consideration of the change of water head under tidal action, through the means of indoor model test, the cement-water glass double slurry commonly used in engineering was selected to study the grouting reinforcement of karst surface soft soil layer. Results show that the shape of plasma veins is various in the tidal semitidal and high tide environments, and the advanced grouting technology has the best reinforcement effect. Results also show that the fixed-point grouting technology at the buried depth of 60 cm has the optimal growth rate in the improvement of bearing capacity and water stability characteristics. The maximum increase of compressive strength of the injected soil layer was 46.8%, and the increase range of water stability characteristics was 773 s. The site adopts the fixed-point grouting technology in the half tide period, and the grouting depth is 2/3 of the average depth of the topsoil as the best reinforcement scheme for the coastal karst soft soil layer.