Complex formation， unconsolidated， chloritized， water sensitive， was encountered when drilling in a gold mineral area in Song county， leading to complex conditions in the drilling process， including borehole shrinkage， collapse， leakage， water outburst， broken drill pipes， sticking， burnt drill tools， so and so forth. In correlation with the drilling experience in adjacent mining areas， this paper presents the drilling difficulties in this area， and analyzes the effect of different kinds of mud， including dispersed mud， solid-free polyvinyl alcohol mud， and low-solid polymer mud. The results show that， （1） High-concentration bentonite slurry has multiple functions such as anti-collapse， anti-leakage， and cementing loose rocks. It is simple to prepare and easy to use， and can be used for drilling in overburden. （2） The dispersed mud cannot meet the demand of the complex strata because of poor inhibition； the solid-free polyvinyl alcohol mud has high anti-caving capacities in the early drilling stage， but insufficient ability to deal with collapse. （3） Field applications of the low-solid polymer mud show that it can provides high inhibitive， good rheology and strong prevention of collapse.