Objective To evaluate the impact of psychological contamination in polygraph examination and explore the corrective and preventive measures. Methods The causes of psychological contaminations of examinee were analyzed and the psychological contaminations were classified. Based on the classification of psychological contamination, improved methods of forming question and evaluating principle were established. Results The improved methods were used in casework and the satisfactory results were achieved. Conclusions Impact of psychological contaminations of examinee can be corrected by using right methods of polygraph test.