The Lower Ordovician Honghuayuan Formation is a set of bioclastic limestone strata distributed in Kaili-Majiang-Danzhai area (in the northeastern part of Qiannan Depression), southern Guizhou. Petroleum exploration is not advancing yet in the discovered Majiang oil-bearing paleoreservoir in this area. On the basis of observing and sampling from Middle to Lower Ordovician outcrops and comparative analysis of thin sections of these samples, it is proved that paleo-karstification, in which both of bural karstification and weathering-crust karstification can be identified, was subjected in this set of limestone strata. According to comparing with these karstified outcrops, it is indicated that the weathering crust karst mainly develops in the Majiang oil-bearing paleoreservoir in the uplift area where it was controlled by uplifting and erosion during Early Ordovician period and the bural karstification develops in other areas except the uplift area. Dissolved pores, holes and joints are abundant in the weathering crust karst zone and the karstification provides a nice condition of forming exellent reservoir for Honghuayuan carbonate rock. Besides, the Honghuayuan carbonate rock that is overlapped by Middle to Lower Ordovician Dawan clastic rock may become a good karst reservoir if it was subjected by bural karstification. The other Honghuayuan reservoirs were ever exposed and destroyed in northeastern part of Qiannan Depression. It is suggested that the direction of petroleum exploration should be put to these Honghuayuan paleokarst reservoirs near the paleo-uplift in the southwestern part of Qiannan Depression.