It’s shown by the study that different hollowing rate in grid suspended ceilings has different validity of photoelectric smoke fire detectors, there is a hollowing rate range of grid suspended ceilings which should based on the real experiment results to make sure the suitable place of the fire detector installation. When the hollowing rate is less than the minimum of the hollowing rate range, detectors installed under the grid suspended ceilings would be sounded first. When the hollowing rate is more than the maximum of the hollowing rate range, detectors installed above the grid suspended ceilings would be sounded first; in the range of the minimum and maximum, we should base on the experiment results to make sure the suitable installation places. The model is proved correct by comparing simulation and test results.