Dependence of the position and light intensity of nonlinear hot image in high power laser system on the thickness of Kerr medium has been investigated theoretically and numerically. Based on the propagation of angular spectrum and Bespalov-Talanov small-scale self-focusing theory, the expression for hot image intensity is obtained without thin medium approximation, and verified by numerical simulation. It is shown that, for a given obscuration, the hot image intensity increases monotonously with the medium thickness for a given input laser power, while it firstly increases to reach a maximum value, then decreases as the medium thickness increases for a given B integral. In addition, it is shown that the thin medium approximation is applicable to hot image analysis when the Fresnel number of an obscuration with respect to the length of the Kerr medium is much more than 1. The analytic results are in good agreement with numerical simulations.