As one of the ultra-precision machining tools, 157 nm laser is an ideal means of surface micro-machining for hard and brittle materials. By establishing the mathematical model, the effects of the process parameters on the etched surface roughness (Ra) are discussed for laser polishing of materials. The roughness calculation formula is derived by using the least squares method. Experimental studies on polishing micro-processing of LED-GaN semiconductor films have been performed using excimer laser with 157 nm wavelength. Through the comparison of multiple sets of theoretical and experimental roughness values, there is an error between them, but the relative errors are all below 15%, and the smallest relative error is only 4%. For 157 nm laser spanning, the ablated surface with lower roughness can be obtained when the laser repetition rate is lower than 15 Hz and the scanning velocity is higher than 0.7 mm/min. The mathematical model can reasonably explain the groove formation mechanism of the etched GaN surface, which provides a theoretical basis to improve the processing quality.