The Middle Ordovician strata are well deposited in the south margin of northern Tarim Basin. Drill data reveale that no long-term exposed unconformity surface exists on the Middle Ordovician strata and the karst reservoirs develop in the strata. Bordered by the pinchout line of upper Ordovician Shangtamu Fm., a burial hill area in the north and an inner area in the south can be divided. Cavernous and fractured+vuggy reservoirs are predominant in the burial hill area. Fractured+porous+vuggy reservoirs are dominant while porous+vuggy and cavernous reservoirs are also common in the inner area. Generally, the development of karst reservoirs exhibits bed-liked in lateral and focuses vertically within the interval of 100 meters below the top surface of Yijianfang Fm., whereas the reservoirs get weak in deveploping extent from north to south and distribute locally along faults. By means of the analysis of cores, thinsections, FMI, hydrologic data and geologic setting, it is shown that the Middle Ordovician karst reservoirs originate considerably from karstification taking place along rock layers. The fact that the overlying upper Ordovician mud-rich strata in the inner area prevented phreatic underground water from flowing back to the surface brings in an unstable phreatic water table under which the slow flow zone provides high potential energy for underground water to flow. It is suggested that the geological setting incluidng paleouplift and slope is the prerequisite of developing karstifiaction, and permeable grainstone and fracture system may form available pathways for diagenetic fluids. The karstification along layers advancesa variety of reservoir spaces.