When a chopped light impinges on a solid sample in an photoacoustic cell, an acoustic signal is produced not only with the fundamental but also with the second harmonic component because of the nonlinear photoacoustic effect. An equation of nonlinear thermal wave beam is presented with nonlinear boundary conditions and solved by using the perturbation approach method under the case that the light source has a Gaussian profile. The Hankel transformation is utilized to attain the first and second order approximation solutions of the equation.The analytic results show that the thermal wave beam of second harmonic still maintains the Gaussian profile with a smaller Gaussian radius than that of the fundamental component and also the amplitude of the second harmonic relates with not only the linear but also the nonlinear thermal parameters which might be expected to provide more information from the sample than the linear one.Basing on these vesults, a new nonlinear paotoacoustic technique would be expected to develop.