Based on the nonlinear effect of photothermal radiometry(PTR), a theoretical model of nonlinear heat conduction along with boundary conditions is developed, from which the representations of the second-harmonic signal of PTR are derived. Furthermore, the general features of the second-harmonic signal are discussed, and all of these are well demonstrated by the experimental results. This nonlinear theory provides a theoretical basis for the applications of the nonlinear PTR technique.