Soybean protein is found in protein adulteration of dairy products recently. In order to improve the oversight system for dairy product and ensure legal rights and interests of consumers, it is vital to build up a method for the indentification and the detection of soybean protein. In this paper, 12 kinds of different sources of soluble soy protein homogeneous slurry was analyzed and compared with the two commodities soy protein with SDS-PAGE method. It was found there were mainly 9 group soy proteins with molecular weight range from 1.90×104 to 8.50×104 in the 14 kinds of soy. Each group has a little difference in molecular weight, the relative difference is not more than 3.3%, and the relative content is basically the same. The soy protein molecular weight 4.96×104, 4.17×104, 3.57×104 and 1.98×104 of the soluble protein fraction can be used to identify soy protein adulteration in milk. The detection limit of 4.97×104 and 3.57×104 was 0.2 g/L/. This method had a good linear relativity (R = 0.9981) in the range from 0.1 to 10 g/L/ and the relative error is 0.63~13.3% within 0.60~8.0 g/L.