The exact transmission route of many respiratory infectious diseases remains a subject for debate to date. The relative contribution ratio of each transmission route is largely undetermined, which is affected by environmental conditions, human behaviour, the host and the microorganism. In this study, a detailed mathematical model is developed to investigate the relative contributions of different transmission routes to a multi-route transmitted respiratory infection. The following transmission routes are considered: long-range airborne transmission, short-range airborne transmission, direction inhalation of medium droplets or droplet nuclei, direct deposition of droplets of all sizes, direct and indirect contact route. It is illustrated that all transmission routes can dominate the total transmission risk under different scenarios. Influential parameters considered include the dose-response rate of different routes, droplet governing size that determines pathogen content in droplets, exposure distance, and pathogen dose transported to the hand of infector. Our multi-route transmission model provided a comprehensive but straightforward method to evaluate the probability of respiratory diseases transmission via different routes. It also established a basis for predicting the impact of individual-level intervention methods such as increasing close-contact distance and wearing protective masks.