In an attempt to investigate the effect of band gap energy and defect states on nonlinear absorption, 80 nm thick amorphous TlGaS2(1-x)Se2(x) thin films with different compositions (0 <= x <= 1) were deposited by vacuum evaporation method. Band gap energies and Urbach energies were obtained from linear absorption spectra. The change in composition led to 0.24 eV increase in band gap energy and 0.32 eV increase in Urbach energy. A theoretical model incorporating one photon absorption, two photon absorption, free carrier absorption and saturation of each absorption process was applied to derive the transmittance in open aperture Z-scan data. The values of nonlinear absorption coefficient and saturation intensity threshold were obtained from the model and two different nonlinear absorption coefficient variation behaviors were observed with increasing Se composition (x) in amorphous thin films based on the opposite contributions of decreasing band gap energies and decreasing Urbach energies on nonlinear absorption.