The invention relates to the field of sea surface oil spill detection, and provides a quaternion-based sea surface oil spill identification method and system. The method comprises the steps of 1, collecting reflected polarized light reflected from a target object at different polarization angles, and converting the reflected polarized light into an electric signal, so as to obtain a polarization image of the target object, 2, obtaining a polarization parameter according to the polarization image, 3, projecting the polarization parameters into an HSV color model, and conducting normalization processing on the polarization parameters, 4, combining the quaternion with a Fresnel formula, and performing oil film identification according to a reflected light quaternion expression, and 5, verifying the equivalence relationship between the quaternion-form Poincare ball and Stokes, marking quaternion expressions of different oil films on the Poincare ball, and realizing identification of various oil films at different positions of the Poincare ball surface by the quaternion expressions of different oil films. Division of an oil film area and a water surface and identification and detection of types of spilled oil can be realized.