The invention discloses a method of calculating evacuation time of people in a building based on a black-box theory. The method comprises steps that 1) the numbers of the people entering and leaving the various entrances and the various exits of the building are monitored in a real-time manner, and the retention amount of the people in the building in a certain time duration is counted; 2) the density of the people in the building is calculated according to the internal space characteristic of the building; 3) theoretical action time required by the evacuation of the people is calculated according to a hydraulic computation formula; 4) total time required by the safe evacuation of the people is calculated according to a people evacuation time formula. The operation engineering of the black-box theory only requires measurement of two parameters, namely, the numbers of the people entering and leaving the various entrances and the various exits of the building, and therefore a problem of difficulty in researching personnel flow rule caused by large building space, complicated internal structure, and high occupancy is prevented, the accuracy of the measurement of the retention amount of the people and the density of the people is improved, and then wide practicability is provided.