The 3000m deep scientific borehole for the Tibet polymetallic deposit is the first 3000m deep scientific borehole in the field of the investigation of solid mineral resources in the Qinghai-Tibet Plateau， and it has created the latest hole depth record in the field of small-diameter solid mineral exploration in the Qinghai-Tibet Plateau. The project is located in the high mountain and deep cut area at 5200m above the sea level in Jiama， Tibet， with harsh natural climate， strict environmental protection requirements for the drilling project， high temperature in the deep strata， and complicated strata characterized mainly by strong water sensitivity， serious breakage and high in-situ stress which leads to extremely drilling difficulty. In view of the environmental protection and the stratum problem， through the experimental comparison of clay， the environmental protection fluid loss reducer， the environmental protection tackifier， the environmental protection inhibitor and so on， the treatment agents with the best performance were optimized； by using the method of range analysis of orthogonal experiment， the influence of each index corresponding to the level and the value of the optimum level were obtained， the range of the optimum formula of drilling fluid was determined； and a high temperature resistant and environment-friendly drilling fluid system has been successfully developed with good field application effects.