Concerning the shortcoming of fog-concentration inspection in the field of image defogging, a scatterplot prior of generalized pixel difference-ratio (GPDR) was proposed and Naka-Rushton fitting function was introduced to inspect the fog concentration. Firstly, the gray scatterplots were built under standard foggy image sets with different scenes so as to extract the prior of GPDR, and the correctness of gray difference-ratio prior was verified based on the degradation model of visual field image. Secondly, the Naka-Rushton fitting function was set up according to constraint prior, and the parameter (n,k) of standard image sets from fitting NR function were calculated, and a lookup table of (n,k) corresponding to the fog concentration in the field of view were established. Thirdly, regression analysis was used to calculate the fitting parameter (n",k") of scatter plot of real foggy image, and the comprehensive correlation coefficient between the parameter (n,k) of the standard image sets and the parameter (n",k") of real image sets were calculated, and the parameter (n,k) whose correlation coefficient exceeded the threshold through searching the lookup table were concerned as the level valuation of concentration inspection. Simulations show that result of this algorithm is in line with the concentration change trend by test for fog image with different concentrations, and simulations also show that correlation coefficients between the results of this paper with PM2.5 can be up to 0.95 by test of samples with different concentrations both in the same scenes and in different scenes. This shows that the algorithm can be used as the fog concentration rating for visual field, and the horizontal comparison test shows that inspection accuracy of the algorithm in this paper can be up to 4.8%.