In this paper, an optimized drilling exploration method, the doubling section method, was summarized after many composite drilling section explorations of buried active fault in urban areas. Operation steps of this method are as follows; Firstly,drill a borehole at each of the two ends of the drilling section to make sure that fault is between the two boreholes, then ,drill the third borehole at the middle of the two holes; and secondly,confirm again the segment where the fault is and drill the next borehole in the middle of it. By repeating the similar practice,the accurate location of fault can be constrained progressively. Meanwhile, this paper also uses a quantitative indicator, the key horizon gradient between two boreholes,instead of stratigraphic throw,to determine the location of buried fault and puts forward two criterions: 1 ) the fault is located between two boreholes if the key horizon gradients between these two boreholes are positive and increase with depth ; and 2 ) the fault is located where the key horizon gradients between two boreholes increase obviously relative to the previous values and that of adjacent segments, besides the increase with depth. While in contrast, the key horizon gradient in a normal fault segment decreases obviously. Application cases show that the method can determine precisely the location of buried active fault.