The aim of this work was to develop an ELISA assay capable of detecting adulteration of goat milk and its products with bovine milk in China. Polyclonal antibodies against bovine β-casein were used and modified by mixing them with lyophilized Saanen goat β-casein (blocking) to improve the specificity of the antibodies. The blocked antibodies recognize some specific epitopes of bovine β-casein with no cross-activity with bovine α-casein, κ-casein and whey protein. The absorbance values of the indirect ELISA showed a linear relationship to the adulteration concentration of bovine milk in the range of 2%–50%, which can be applied to quantify adulteration of bovine milk. There was no significant difference in titration curves between Saanen and Guanzhong goat milk, and the detection limits were all 2% for goat caseins, raw milk and heat-treated milk. Low coefficients of variation (<10%) of the experiments indicated good repeatability. The ELISA assay is likely suitable for routine detection of cow’s milk adulteration of Shaanxi goat’s milk.