Bi₂S_3-xSe_x/poly(methyl methacrylate) (PMMA) nanocomposite films were prepared using microwave assisted synthesis with different compositions of x. Crystal structure, surface morphology, and optical properties were investigated to characterize the prepared nanocomposite films. The crystallinity and optical band gap of the prepared Bi₂S_3-xSe_x/PMMA were affected by x. The prepared samples showed a blue shift in the absorption edge. The laser power dependent nonlinear refraction and absorption of Bi₂S_3-xSe_x/PMMA nanocomposite films were investigated by using the Z-scan technique. The optical nonlinearity of the nanocomposite films exhibited switchover from negative nonlinear refraction to positive nonlinear refraction to negative nonlinear refraction effects, and from saturable absorption to reverse saturable absorption to saturable absorption with an increase and decrease in the composition. An interesting all-optical figure of merit was reported to assess the nanocomposite films for a practical device. It was calculated that the device all-figures of merit were based on the nonlinear response, which is important for the all-optical switching device. The results demonstrate that the optimized all-optical figures of merit can be achieved by adjusting the composition and input laser power, which can be used for the design of different all-photonic devices, and the results of nonlinear switching behavior can open new possibilities for using the nanocomposite films in laser Q-switching and mode-locking.