Femtosecond laser ablation is one of the high precision machining methods,and the formed defects in the machining process have great impact on its applicability.The experiments of femtosecond laser ablation of polystyrene were carried out.The formation mechanism of defects in the process of laser ablation was investigated,and the impact of parameters such as laser intensity,speed and environment on the formation of defects was analyzed.Finally,the optimal machining conditions were obtained.The study has important reference value for femtosecond laser abaltion of polystyrene.