Under Rytov approximation and geometrical optics approximation, a formula of the variance of the log-amplitude derivative was deduced for the case of a plane wave propagating through turbulence. It was clarified that the main factors, which determined the variance, were the Rytov variance, the turbulence inner scale and the Fresnel size. And, based on the formulae deduced by Voitsekhovich, the expression for density of phase branch points was modified. The relationship between the density and the above mentioned parameters was analyzed thoroughly, which indicates that the density increases with Rytov variance increasing and decreases with turbulence inner scale and Fresnel size increasing under the condition of Rytov variance less than 1.