The characteristics of groundwater flow in karst area is complicated. This paper took the Mengzi fault basin as a case to reveal the flow directions of the karst groundwater in different hydrogeological units based on hydrogeological investigation, tracing experiment and comprehensive study. The results show that karst groundwater flow from southeast to northwest in the units of Dongshan –Mengzi (I-1), Datun (I2) and Heilongtan (I-3) in the Mengzi basin, from south to north in the unit of Xin’ansuo (I-4), and from north to south in the 2 units of No.8 underground river (II-1) and No.9 underground river (II-2).