In trenchless horizontal directional drilling in high-altitude areas， mud leakage in fracture-developed formation is one of the key problems that plagues trenchless works. It will not only affect the effective circulation of drilling fluid， but also form a large number of cuttings beds in the borehole. That will lead to other problems such as excessive wear of drilling tools， abnormal increase in frictional resistance， and reduction of the effective diameter. In view of the granite fractures in a Tibetan site， the close-range photogrammetry technology was used to quantitatively characterize the fractures， and found that the fracture width of the fractured strata in the study area was less than 4 mm. At the same time， basalt fiber is proposed as the main fracture plugging agent， and the formulation system of basalt fiber plugging-while-drilling has been established. The experimental results showed that the multi-grade combination of 3mm， 6mm and 9mm basalt fibers can achieve the best results， where the 3mm fiber can form a space skeleton structure with inert materials， and the 6mm and 9mm fibers can function as “reinforcing” with a certain strength. Results at the jobsite indicate that the solid phase content in the mud decreased by 13.14% with basalt fiber plugging， ensuring effective， green， and safe drilling for the long-distance mountain crossing with trenchless technology.