An investigation is made on the optical nonlinear characteristics of MgF₂ films containing Cu nanoparticles. When the Cu volume fraction is 0.1, at approximately \varepsilon_m+ 2\varepsilon_d= 0, the film exhibits a strong third-order optical nonlinear refractive coefficient of 1.0×10^-9 cm²/W, a nonlinear refractive index of about ?4.0×10^-7 esu, a third-order optical nonlinear response of about 6.8×10^-8 esu, and a figure of merit of about 1.0×10^-11. The result shows that the film may have potential applications in optoelectronic technology.