The calculation of evacuation time is an essential part of building evacuation safety design. Some investigations demonstrate that the effect of pre-evacuation time on evacuation time is highly significant. Therefore, the pre-movement time in evacuation time during a performance-based design should be considered. However, the essential data related to pre-evacuation time of public buildings in the case of fire is quite scarce. By carrying out an emergency evacuation experiment in classroom under fire-simulated condition, students' pre-evacuation time of assembly occupancy of school is focused on. Using video camera, a simulated evacuation process is completely recorded. The pre-evacuation time is defined as the time from smoke induced by tobacco spread to the classroom to the beginning of evacuation. With analyzing every one in the experiment, the data concerning with pre-evacuation times are collected. The statistical results of students' pre-evacuation times demonstrate that normal distribution or other distribution functions used in the previous publications are unable to characterize the students' pre-evacuation times. Using cumulative distribution function to express the pre-evacuation time is proposed and the cumulated distribution curve of pre-evacuation time is obtained by applying the method. In addition, the cumulated distribution of stochastic data generated according to the cumulative distribution function fits quite well with that of the experimental data.