Abstract: In the case of fire in the sealed cabin of the manned spacecraft, the movement of the smoke under the microgravity condition is different from that under the normal gravity condition, which increases the difficulty of spotting and extinguishing the fire timely. In this paper, the FDS software is used to simulate the fire smoke spreading under the influence of the cabin ventilation for an imaginary double fire source located on the bottom, the sidewall and the upper surface of the sealed cabin of a manned spacecraft. The results indicate that the position of the double fire source has a significant influence on the distribution of the smoke concentration in the cabin. When the double fire source is located on the sidewall of the cabin, the smoke is accelerated in the vertical direction due to the influence of the swirling airflow. Therefore, it is recommended to be near the high heat consumption instrument and the return air inlet. The polynomial function is used to fit the variations of the average smoke concentration in the typical time period when the double fire source is located on the side and the top surface under ventilated conditions. The calculated result from the obtained fitting formulas conforms well to the result of the simulation. And it can accurately reflect the change of flue gas concentration.