In order to solve the difficulty in drilling holes in fractured complex rock strata due to wall collapse in underground directional drilling at coal mines， a casing while directional drilling technique for complex rock strata was invented to protect the hole section over complex fractured rock. First， the concentric directional drilling stem drills to pass through the complex strata with the outer casing； then， the concentric directional drilling stem is tripped out of the hole and the outer casing is left in place to protect the hole section over the complex strata， and finally， the directional drilling stem of smaller size is run through the outer casing to continue drilling. In this paper， the design of the casing while directional drilling tool and the research on the casing while directional drilling process are described. The technique was tested for drilling a high-position directional hole at a mine in Huaibei， where it successfully protected the 103.5m complex strata hole section composed of two layers of coal lines and one layer of mudstone with the completion depth of 530m. The test showed that this technology can effectively protect the complex strata hole section， and increase the directional drilling depth in complex strata and protect the extraction channel.