Due to its noninvasiveness, high resolution, and high sensitivity, photoacoustic imaging has developed rapidly in the field of biomedicine. However, research on dermatosis detection by photoacoustic imaging is still lacking. In this paper, the skin is modeled as a multilayer planar medium based on the non-homogeneous, complex layered structure of the skin tissue. Then, the analytical expression for the photoacoustic signal of multilayer skin tissue was derived under the assumption that the thermal and optical parameters of the skin tissue do not vary with temperature. The expression not only considers the influence of optical, thermal, and mechanical parameters of the tissue on the photoacoustic signal but also, for the first time, the influence of the number of skin layers on the photoacoustic signal. The analytical expression of the photoacoustic signal containing the number of skin layers is also given. The numerical simulation results show that the difference between the photoacoustic signal of the seven-layer skin model and the single-layer skin model is 15.206 × 10−6 MPa when ω = 3.5 MHz and μ a = 2.70 cm−1. Therefore, the increase in the number of model layers enhances the amplitude of its photoacoustic signal. This work provides a comprehensive study of photoacoustic mechanisms in dermatosis tissues and establishes a theoretical foundation for the application of photoacoustic imaging detection technology in the diagnosis and treatment of dermatosis, which may improve treatment plans.