In this paper, we set up the model of Gaussian distribution on the temperature of illuminated superconducting film and derive a serial formula, from which the equation of the substrate thermal conductance G3= 2πks· d · y(a(t),b(t),r) can be obtained. It can be used not only for G of single-layer substrate, but also for G of multi-layer substrate. The calculated results are in good agreement with the experimental ones.Using Maths CAD programm, the relations between illumination radius r, substrate thickness d or frequency f and G3 could be obtained. Based on the above, the optimized structure or art parameters of the detector can be selected and some effective methods for improving its properties will be considered.