Low excitation-power density photoluminescence (PL) spectra suffered from poor spectral signal-to-noise ratio (SNR)and hence restricted the study of the band-tail states in GaAs1-xBix. We conduct laser spot-size-dependent PL measurements on two GaAs1-xBix epitaxial films by Fourier transform infrared spectrometer-based PL system with enhanced sensitivity. It is observed that (i) with constant excitation power, increase of laser-spot diameter leads to a redshift of the PL-peak position and a linewidth evolution of first decreasing and then increasing, which is attributed to the decrease of the equivalent excitation power density, and (ii) with a constant excitation power density of 5.1 W/mm2, the PL lineshape is unchanged while the SNR is significantly improved as the laser spot diameter rises. As a result, the weak transition features are well resolved in the PL spectra taken at low excitation power density, and can be safely treated by spectral fitting analysis. The result indicates that adequate increase of excitation spot-size contributes to the improvement of the SNR and sensitivity as well for PL measurement at low excitation-power density.