The invention provides a subway ticket evasion preventing system based on an ultrasonic sensor and a design method of the subway ticket evasion preventing system. The system, on the basis of not changing existing equipment, is added with the ultrasonic sensor; pedestrian intervals and the number of pedestrians in an entrance channel are judged by acquiring distance data; with help of a card swiping number counter, the system can judge a ticket evasion phenomenon and open or close a gate. Through the ultrasonic sensor and a single chip program processing system, the system disclosed by the invention can automatically detect the ticket evasion phenomenon, overcome shortcomings of the existing equipment and transmit an alarm signal, therefore the accuracy and the intelligence degree of entrance automatic ticket checking are enhanced; the system disclosed by the invention saves human, physical and financial resources, prevents common occurring of the ticket evasion phenomenon and avoids unnecessary mistakes in a manual supervision process; through the ultrasonic sensor, the system solves the difficulty of manually supervising ticket evasion, and has advantages of being simple, efficient and accurate; the system well realizes electronization, intelligence and automation of a subway system, and can be commonly popularized on the basis of an existing subway system.