The influence of process parameters including punch frequency, needle penetration depth and punch times on the fiber distribution and straight segment fiber length of needle punched nonwovens was investigated using the Hough transform method. The results show that punch frequency and punch times have more significant influence on the samples than needle penetration depth. When combining the experimental results with image analysis, the optimum process parameters to prepare needle punched nonwovens based on preoxidized polyacrylonitrile were obtained.