A chemically modified poly(fluorene-alt-benzothiadiazole) (PFBT) polymer film is reported to exhibit high two-photon absorbing capability and chemical/physical stability upon the action of high-power laser pulses of $\sim 780\mathrm{nm}$ wavelength and $\sim 160\mathrm{fs}$ duration. A nonlinear transmission measurement is conducted by varying the input intensity from $\sim 20$ to $\sim 600\mathrm{GW}/{\mathrm{cm}}^2$ , the corresponding nonlinear transmission of a $\sim 70\mathrm{\mu m}$ thick film is reduced from $\sim 0.8$ to 0.18, indicating a superior optical limiting behavior. In the meantime, intensity fluctuation of laser pulses can be significantly reduced after passing through the same film sample. Based on the intensity-dependent nonlinear attenuation mechanism, a straightforward optical reshaping effect on spatio-temporal profiles of the laser pulses has also been demonstrated.