We carried a detailed spectral and temporal study of blazar PKS 0903-57 using the Fermi-LAT and Swift-XRT/UVOT observations, during its brightest flaring period MJD 58931–58970. During this period, the maximum daily averaged γ-ray flux (F0.1−500GeV) of 9.42 × 10−6 ph cm−2 s−1 is observed on MJD 58951.5, the highest γ-ray flux detected from PKS 0903-57 till now. Several high-energy (HE) photons (>10 GeV) consistent with the source location at high probability (>99 percent) are detected, and the γ-ray light curve in the active state shows multiple substructures with asymmetric profile. In order to understand the possible physical scenario responsible for the flux enhancement, we carried a detailed broad-band spectral study of PKS 0903-57 by choosing different flux states from its active period. Neglecting the multiband variability in each of the selected time intervals, we could reproduce their averaged broad-band SEDs with a one-zone leptonic model whose parameters were derived with a χ2 -fit. We found that the broad-band SED during different flux states can be reproduced by the synchrotron, synchrotron-self-Compton (SSC), and external-Compton (EC) processes. The seed photons for EC process from BLR or IR torus provide acceptable fits to the GeV spectrum in all the flux states; however, the detection of HE photons together with the equipartition condition suggests that the EC/IR process is a more likely scenario. Further, a detailed comparison between the fit parameters shows that the flux enhancement from quiescent-state to the flaring-state is mostly related to increase in the bulk Lorentz factor of the emission region and change in the break energy of the source spectrum.