Spatial averaged depolarization requires that depolarized light beam not only has a low degree of polarization(DOP), and also avoids the polarization state exhibiting continuous band in spatial distribution.To achieve the measurement of the DOP of the beam and observation of the degree of spatial depolarization, a method based on the Rochon prism was proposed.The dependence of the DOP on each optical power measurements was firstly deduced, following with experimental measurement and comparison to full polarization test method.The relative deviation in the DOP for the same light beam, 0.192%, shows the two methods have the same performance in measurement of DOP.However, the feature of the proposed method separating o component and e component in the space enable observations of the spatial distribution of polarization states respectively.The Rochon prism based method will play an important role in simultaneous measurement of DOP and observance of spatial polarization distribution.