This paper presents a second harmonic photoacoustic microscopy (SH-PAM)for subdiffraction-limited imaging based on nonlinear thermal diffusion. When biological tissues are irradiated by intensity modulated Gaussian laser beams, the temperature field of Gauss distribution is formed by absorbing photons in tissues. Considering the nonlinear photoacoustic (PA)effect caused by the nonlinear thermal effect of thermal diffusivity, the second harmonic photoacoustic (SHPA)signal is generated. The simulation and experimental results show that the lateral resolution of reconstructed photoacoustic second harmonic imaging is higher than that of traditional optical imaging. In this paper, the feasibility of this method is verified by bionic samples, and the imaging of human skin epidermal cells is carried out to prove its imaging ability for biological samples. This method expands the range of conventional photoacoustic imaging, opens up new possibilities for super-resolution imaging, and provides great potential for biomedical imaging and material detection.