The process of preparing active carbon with brown coal in supercritical water includes filling brown coal into filter and setting in reactor, heating to 500-700 deg.c, pumping in water preheated to 300-650 deg.c into the reactor in the rate of 8000-12000 kg/sq m.hr, raising pressure to 22-40MPa to react for 0.5-3 hr, taking out solid reaction product and soaking in HCl solution of 3-10 vol% at 60-100 deg.c for 10-40, and drying to obtain the active carbon product. The process can utilize brown coal with high water content directly, and has the advantages of lower reaction temperature, short reaction period and no need of pre-treating ash content in brown coal.