The invention provides an evaluating method of the protective effect on a gravel filling sand-prevention reservoir layer and belongs to the technical field of marine oil drilling and production. The evaluating method is mainly used for evaluating the protective effect of a reservoir layer in the gravel filling sand-prevention operation of an oil-gas well. In the evaluating method, a rock core, a gravel filling layer and a screen tube are effectively combined, an existing rock core return permeability evaluating device is improved, the protective effect of the gravel filling sand-prevention reservoir layer is evaluated, and an evaluation standard is built. The combined influence degree of the stratum obtained after the gravel filling operation, a drilling fluid filter cake, the gravel filling layer and the screening tube on the protective effect on the reservoir layer can be truly and effectively evaluated, the operation is simple, the result is accurate, and the repeatability is high.