In this paper, the nonlinear optical effect of a sandwichlike optical switch composed of nonlinear media with negative refractive index variation and light irradiation is discussed. Starting from its fundamental theorem, we analyzed the mechanism of light-induced refractive index variation and inferred that the light-induced thermal effect is the major cause of the refractive index variation. Then, we discussed in detail the factors which influence the time response characteristics. The theoretical analysis agrees with the experimental results.