Abstract: Borehole instability and reservoir damage have become an international technical problem in petroleum exploration and development of complicated area. The main reason for borehole instability and reservoir damage is caused by invading of water phase from drilling fluid into borehole wall and oil reservoirs. In order to prevent the water of drilling fluid invading borehole wall and oil reservoirs, the technology of ultra-low permeable drilling fluid has been put forward recently by the domestic and foreign scholars. Film forming agent is the key treating agent of ultra-low permeable drilling fluid. In this article, the film forming agent HN-1 is introduced. This film forming agent has no great influence on the other properties of the drilling fluid, and can decrease the filter loss of drilling fluid applied in Enping 24-2 oil field. Therefore, the ultra-low permeable drilling fluid applied in Enping 24-2 oil field has been formulated. With the testing methods of drilling fluid properties for ultra-low permeable reservoirs, evaluation has been conducted on the invasion depth of drilling fluids into sand-beds, drilling fluid filter loss at high temperature/high pressure sand-beds, the pressure-bearing capability of cores, and the ability of reservoir protection. The results showed that the film forming agent HN-1 decreased drilling fluid invasion sand-bed depth and drilling fluid HTHP sand-bed filter loss, improved the pressure-bearing capabil-ity of core and the capability of reservoir protection, which provides an efficiency guarantee for the development of Enping 24-2 oil field.