Abstract: Combining Monte Carlo laser-Compton scattering X/γ-ray simulation program 4D-MCLCSS and Geant4 toolkit, the γ-ray production and the induced photo-transmutation of ¹³⁵Cs using Shanghai Laser Electron Gamma Source (SLEGS) were modeled. The dependence of photon-transmutation rate on the ¹³⁵Cs target geometry parameter was predicted. The distribution of transmutation product of ¹³⁵Cs was also investigated. It is found that an optimal thickness and radius for ¹³⁵Cs target are 55 cm and 0.6 cm, respectively. Accordingly, the photon-transmutation rate is calculated to be 1.62×10⁶ s^-1, which is an order of magnitude higher than that by laser-driven bremsstrahlung sources. It is suggested that the SLEGS facility is useful for photon-transmutation of long-lived fission products, such as ¹³⁵Cs.