In laser machining such as drilling with $CO_2$ or Nd:YAG laser, and etching or ablation with Excimer laser, one of the most important parameters affecting the machining is known to be beam characteristics. In this paper a numerical study is performed to investigate the effects of beam parameters, especially in the process of excimer laser ablation of polymers. Results of different beam conditions reveal that if the ablated depth is small compared to beam size the simple photochemical etching model is suitable to predict the etched shape, and that the importance of precise alignment becomes large as beam quality factor becomes larger.