Abstract: Aiming at the deficiency of the similarity parameter in applications to measure the degree of scattering similarity between distributed targets and canonical targets， a new parameter， namely polarimetric scattering similarity， is proposed based on the Pauli vector of a canonical target and the coherency matrix of a distributed target in this paper. Since this parameter contains all similarity information between the scattering mechanisms of a distributed target and canonical mechanism， it describes accurately the degree of average scattering similarity between a distributed target and a canonical target. Furthermore， this new parameter is not only independent of target sizes， but also independent of the target orientation angles just like the similarity parameter. As an application， the parameters to several canonical targets are derived and their applications in terrain classification are also discussed with AIRSAR data.