X-ray fluorescence is an effective method monitoring the heavy metals successively. To monitor the atmospheric heavy metal in factory on-line, sample enrichment and accurate control of distance between the X-ray detector and the sample are essential. An accurate distance control method, with a photoelectric displacement sensor monitoring the shift of sample, is proposed. With an MCU, the distance control system is worked out. Using the distance control system, a series of sample location control experiment are finished. The relative standard deviation of data is about 0.5%. The deviation is less than 0.8 mm between the actual distance and the exact distance of 100 mm. It is confirmed that this distance control system can afford the sample location requirement of the on-line monitor of the atmospheric heavy metal.