In this article, the distributions of droplet nuclei with pathogens which was evaporated from the droplets released by cough was and trace gas CO2 were compared to study the airborne characteristics of droplet nuclei. The infectious isolation room model with steady particle source model was built. The particle source was built according to the size distribution of the droplet nuclei with pathogens which was evaporated from the droplets released by cough. The CO2 source was build at the same location with the particle source and released in the same way. The simulation result showed that the distributions of droplet nuclei and CO2 which were simulated by CFD were similar under the same ward airflow. This proved that these droplet nuclei could suspend in air and move with airflow. it was feasible to use tracer gas CO2 as trace gas to solve the validating the numerical simulation of droplet nuclei distribution in infectious isolation room.