The precise definition of polarization transform is proposed from the viewpoint of polarization domain transform for the first time. The mathematical principle,char-acteristics and physical meanings of polarization transform are analyzed. Two typical kinds of polarization transforms which are called unitary matrix polarization transform (UMPT) and diagonal matrix polarization transform (DMPT) are put for-ward. The gain property and the change property of polarization state distance are presented in detail. Thereby,the concept of serial polarization transform is pro-posed. A general polarization transform can be realized by two steps: changing the location of polarization states in polarization domain by UMPT; and expanding or compressing the density of polarization states by DMPT. The result is meaningful to many applications including polarization filtering,detection,enhancement,anti-jamming,discrimination and so on. The precise definition of polarization transform is proposed from the viewpoint of polarization domain transform for the first time. The mathematical principle, char-acteristics and physical meanings of polarization transform are analyzed. Two typical kinds of polarization transforms which are called unitary matrix polarization transform ( UMPT) and diagonal matrix polarization transform (DMPT) are put for-ward. The gain property and the change property of polarization state distance are presented in detail. Here, the concept of serial polarization transform is pro- posed. A general polarization transform can be realized by two steps: changing the location of polarization states in polarization domains by UMPT; and expanding or compressing the density of polarization states by DMPT. The result is meaningful to many applications including polarization filtering, detection, enhancement, anti-jamming, discrimination and so on.