Abstract: Accurate and reliable online monitoring of coal quality is a crucial link in maintaining consistency of saleable coal product, increase of yield of clean coal and effecting intelligent control of coal washing process. In order to realize online monitoring of ash of products of heavy medium separation system, an online ash monitoring system based on the principle of prompt gamma neutron activation analysis(PGNAA) is developed. As evidenced by the test result, for monitoring the ash of the product of coarse coal heavy medium washing system, the dynamic standard deviation of the system is as low as 0.105% which is 0.18 percentage point lower than the figure obtained through manual control method. In addition, the yield of clean coal is seen to rise by 0.95 percentage point.