The invention relates to a sec butyl alcohol extraction based method for extracting total DNA from soy. The invention also relates to a method for detecting soybean gene in soy. The method comprises a NDA obtaining step through the above method. According to the method of the invention, a soybean DNA can be extracted, with high yield and high purity, from an extreme small amount of sample; besides the method can facilitate monitoring of transgene soybean used in soy foodstuff through a molecular biological experiment method, so as to ensure foodstuff safety.