If only the first order correction to the scalar paraxial field is concerted, then the field can be expressed as =[ε x,0,ε z] and the longitudinal field energy E z is proved to be constantly independent of the position z of the beam cross section. The value of E z stands for the degree of scalar paraxial approximation. An approximation factor η is suggested to characterize this property. The larger η, the worse degree of the approximation. Results are applied to super Gaussian beams.