The assessment of optical surface flaws requires a standard which objectively reflects their influences upon an optical system and which is also widely accepted. Can the area of flaws, which has been used in most national standards, reflect objectively the performances of optical surface flaws? In this paper, calculation and analysis of the frequency spectrum distributions of typical surface flaws (including scratches and digs) in an optical system have been performed. The relationship between the spectrum intensity distribution and flaw area, depth, and shape has been given. The corresponding experimental results of several 20 micrometers wide scratches with different depths or cross-section shapes have been obtained. From the results obtained above, a novel method which is called Spectrum Energy Function Assessment has been put forward. This method is based on the performance of surface flaws (i.e., spectrum energy) instead of the area of flaws which is widely used in traditional assessment method. The comparison between the two methods has also been described. The results show that the performances of two surface flaws, which are thought to be of the same quality based on the traditional method, are greatly different in the same optical system obtained from the experimental results.