Two-dimensional (2D) layered materials with excellent properties such as strong spin-valley coupling, high current on/off ratio and carrier mobility, have gained great attentions in optoelectronic applications including photodetectors, field effect transistors and light-emitting diodes. Meanwhile, many Novel Nonlinear Optical (NLO) phenomena such as saturable absorption, reverse saturable absorption and two-photon absorption have also been observed. Based on these nonlinear absorption behaviors, ultrafast photonic applications including Q-switching, mode locking and optical limiting have successfully been designed in 2D materials. The device performance is strongly dependent on their thicknesses due to tunable electronic properties with layer. For example, giant two-photon absorption can be observed in monolayer WS₂ film with 1.9 eV while excellent saturable absorption has been demonstrated in multilayer. However, the related research about NLO nonparametric processes with layer thickness, which is crucial to design high performance photonic devices, was seldom reported due to uncontrollable thickness in large-area. Therefore, seeking for an efficient way to produce large-scale and controllable film is highly desirable. To date, mechanical exfoliation have been developed to acquire few-layer nanosheet through exfoliating bulk material layer by layer. However, these methods are low reproducibility and low yield and even limit their application in photonic devices. Compared with mechanical exfoliation method, liquid phase exfoliation was quickly developed and most 2D materials were successfully exfoliated to few-layer or monolayer nanosheet. The nanosheet dispersion can be used to form wafer-scale film by vacuum filtration technique, which provides an effective way to prepare large-area film. Herein, WS₂ nanosheet dispersion was prepared by liquid phase exfoliation and then WS₂ films with different thicknesses were prepared by a vacuum filtration method with different volumes. The atomic force microscopy confirmed the nanosheet length is between 2 and 4 μm and the thickness of thin film is 50 nm for 30 mL in fltration volumes. Raman spectroscopy was used to charaterize the sample composition and the typical Raman peaks confirmed the preparation of WS₂ films. UV-Vis spectra was used to obtain the linear absorption and vetified the defects in the grain boundary. The third-order NLO absorption of prepared WS₂ nanofilms was studied by a Z-scan technique based on 800 nm femtosecond laser. It is found that all WS₂ with different thicknesses show the characteristics of saturable absorption, which is mainly due to the Pauli blocking effect caused by single photon absorption. To confirm the absorption process, the electronic structure was calculated by density functional theory and the calculated bandgap of bulk WS₂ is approximately 0.8 eV. The photon energy is higher than the bandgap, which vertified the single photon absorption occurs. The nonlinear absorption coefficients are -150 cm/GW, -139 cm/GW, -133 cm/GW, -122 cm/GW, and -115 cm/GW from 30 mL to 70 mL with per 10 mL fltration volumes. Obviously, the absolute value of nonlinear absorption coefficients decreases with the incereased thickness. This decrease is mainly due to the reason that thicker films with high density defects capture more photoexcited carriers and generate more nonlinear scattering as well as energe loss, resulting in the dependence of third-order nonlinear optical absorption with thicknesses. The existence of certain defects will introduce defect energy levels, which has a great potential in the application of infrared and near-infrared Q-switched and mode-locked lasers. With the increase of thickness, the saturation intensity is basically unchanged, and the modulation depth increases while the absolute values of the imaginary part of the third-order nonlinear optical susceptibility and figure of merit decrease. Our results provide provide experimental support for the applications of WS₂ thin films in ultrafast switchers and ultrafast lasers.