The fire dynamics software FDS was used for fire in large space buildings to carry on the numerical simulation, and the different sizes of three kinds of fire - cotton rope smoldering fire, n-heptane flame and shredding smoldering fire were explored for their respective smoke movement law, and the response characteristics of linear infrared beam smoke detector were analyzed. Through simulation, different fire smoke movement law was found, and there was a significant difference between the fire sources, fire scale and installation position of detectors, which will all affect the response time of the fire detector.