Abstract: The scope of the impact of foreign objects invasion is determined by disaster prevention design department, which provides exact data to signal designed department. From the point of view of disaster prevention, the greater the range of the foreign object invasion monitoring, the better it is. However, from the point of view of signal speciality, the excessive range of monitoring may lead to a larger designed impact area compare with the fact, thus would influence traffic efficiency and even endanger traffic safety. This article analyzed the impact of range of foreign object invasion monitoring on processing of train control system in highway overpass railway, put forward the correlation between them through formula calculation, proposed several reasonable suggestions from the angle of disaster prevention and signal.