The bus with air-condition was chosen as research object in order to analyze the dispersion of droplet in bus and reveal the mechanism of virus transmission. The dispersion characteristic parameters of droplet such as dispersion path, concentration distribution and escape rate were obtained by the analysis of flow field. The calculation of transport of the droplets exhaled from a single cough by patient in the front, middle and back of the cabin with the Lagrangian method. Then the infection risk for passenger can be evaluated by the Wells-Riley equation according to the amount of inhaled virus particles by passenger. Results show that location of the patient and air return opening, the airflow pattern can significantly affect the droplet distribution. The patient in the front may cause more passengers at a higher infection risk compared with the patient in the back. The infection probability of passengers located in the droplet dispersion pathway and within three rows of the patient is higher than 30%.