The aim of detecting pulmonary nodules by computer-aided diagnostic system is reduce doctors' workload. Firstly, the part of pulmonary parenchyma should be extracted from a CT image. To extract the pulmonary parenchyma's contours well, we can utilize mathematical morphology filter. Then, Snake model is used to extract suspicious nodule's contours in pulmonary parenchyma. Finally, program will eliminate interference automatically based on the pathological features of pulmonary nodules and mark these suspicious nodules on CT image.