In view of low success rate and slow speed in RIH of over-long casing in underground directional boreholes in coal mines， technical improvement measures are put forward， such as straight-maintaining drilling in the early section， control of curvature in the deflecting section， lowering of the kelly bar with drill rig， centralization of casing. Field test was carried out in Xinjing Coal Mine of Yangquan， where over-long casing was run in three directional boreholes for underground hydraulic fracturing with the maximum depth up to 168m. The success rate of casing running reached 100%， and the average casing running efficiency increased from 5.5m/h to 13.1m/h， eliminating effectively the limitation of over-long casing running in underground directional boreholes in coal mines.