Abstract: Low-alloy engineering structural steel is widely used in many fields, because of its good mechanical and processing properties. The corrosion resistance of low-alloy engineering structural steel is related not only to chemical composition but also to microstructure, inclusions, grain size, and other factors. However, at present, the direct and fast criterion for evaluating the corrosion resistance of low-alloy structural steel, i.e., the corrosion resistance index I, in the ASTM Standard and China National Standards only involves the chemical composition of low-alloy structural steel and ignores the microstructure, inclusions, and grain size. In the literature on the corrosion resistance of low-alloy structural steel, the coupled effect of chemical composition and other material factors on corrosion resistance and the quantitative analysis of each factor have not been reported. Therefore, a new corrosion resistance index for low-alloy structural steel that includes more factors needs to be proposed. Through the neutral salt spray accelerated corrosion test of eight kinds of low-alloy engineering structural steels with similar composition and microstructure produced by different manufacturers or production lines, combined with the methods of composition test, microstructure analysis, corrosion product analysis, data statistics, and calculation fitting, a composite corrosion resistance index Y for low-alloy structural steel was proposed, and a quantitative index formula containing several factors, including chemical composition, inclusions, microstructure, and grain size, was established. Results show that the corrosion resistance of low-alloy structural steel is affected by the coupling of many material factors, not only the traditional corrosion resistance index I but also inclusions, microstructure, and grain size. The degree of influence is in the order of corrosion resistance index I determined by corrosion-resistant alloy elements, total inclusions, pearlite content, and grain size. The composite corrosion resistance index can be used as an effective criterion for the corrosion resistance of low-alloy structural steel and is of significance in engineering applications.