Abstract: Epoxy-coated reinforcement often exhibits inadequate adhesion to the concrete’s protective layer, compromising the overall structural stability. In contrast, polymer cementitious coatings demonstrate superior bonding to rebar compared to epoxy coatings. By employing polyacrylate latex (PA) and P·O 52.5 cement, this study explores the influence of different mass ratios of polymer cement-based coatings on engineering performance. Various tests, including crosslinking degree assessment, coating pull-out test, chloride ion permeability test, and Machu test, are conducted to investigate the impact mechanism of polymer-cement mass ratio (poly-ash ratio) on coating properties. Scanning electron microscopy-energy spectroscopy (SEM-EDS) is utilized to analyze the interaction mechanism between the polymer and cement hydration products. The comparative tests reveal that the polymer-cement ratio significantly affects the engineering performance of the coating, with an optimum range observed at 22% to 27%. Deviating from this range, either by exceeding or falling below it, diminishes the film’s strength and compactness, thereby weakening its engineering performance. Scanning electron microscopy demonstrates that the mechanism by which propylene latex cementitious coatings enhance corrosion resistance and engineering performance of steel bars lies in the polymer’s ability to fully crosslink into a film and gradually envelop the cement hydration product, forming a continuous film. This process improves the coating’s compactness and overall engineering performance. In conclusion, by adjusting the poly-ash ratio of polymer cementitious coatings, it is possible to produce superior rust-inhibiting coatings for steel reinforcement.