In this paper, based on the autocorrelation coefficient of the transmittance function of the diffractive optical element (DOE) for beam smoothing, two performance parameters, light efficiency and top non-uniformity, are re-defined. This definition is a kind of approximation of that defined by the spatial frequency spectra of the far filed intensity distribution of the DOE for beam smoothing. The simulated results calculated with two definitions are almost same for the precise designed DOE, but different for the traditional designed DOE. When smoothing by spectral dispersion (SSD) is used with the DOE to realize beam smoothing, the simulated results show that the definition in this paper can reflect the practical performance of the DOE for beam smoothing, no matter whether the DOE for beam smoothing is designed with the precise design method or the traditional design method.