The invention relates to an intrusion detection method based on a fast k-nearest neighbor (KNN) algorithm. The method comprises the following steps: carrying out data pretreatment; receiving original data of intrusion detection; carrying out data pretreatment on the original data, wherein data pretreatment includes continuous data discretization treatment and character data digitalization treatment; reducing features; carrying out feature reduction on the pretreated data by adopting a mutual information-based feature reduction algorithm; detecting in a classified manner; carrying out classified detection on the data after feature reduction by adopting the fast KNN algorithm, and outputting the classified result. Compared with the prior art, the intrusion detection method has the advantages of high classification effectiveness, high classification accuracy and the like.