Abstract: As the oil and gas exploration and development gradually shifts to complicated formations such as deep formation, super-deep formation, deepwater formation, high-temperature high-pressure high-sulfur-content formation and multi-pressure formation, the problem of lost circulation becomes more and more serious, which greatly delays the progress of oil and gas exploration and development. Therefore, researching the diagnosis of leak-off of drilling fluid and revealing the dynamic behavior and characteristics of leak-off of drilling fluid are of great significance for understanding the lost circulation and optimizing the leakage prevention and sealing technologies. The flow model for leak-off of H-B flow pattern drilling fluid in 2-dimensional planar fracture has been established, and the dynamic behavior and affecting factors of leak-off of drilling fluid have been revealed. The research results indicate that, as the aspect ratio, area, extended length, deformation and surface inclination angle of 2-dimensional planar fracture increase, the leak-off rate and accumulative loss quantity of drilling fluid will increase. As the consistency factor and dynamic shearing force of drilling fluid increase, the leakage loss rate and accumulative leakage quantity of drilling fluid will decrease. The dynamic evaluation instrument for leak-off of high-temperature high-pressure drilling fluid has been used to test and evaluate the leak-off of drilling fluid in planar fracture of which the width is 0.5 mm and 1 mm respectively, and the result is consistent with the result simulated by leak-off model, with the error being less than 25%. This indicates that, the established flow model for leak-off of drilling fluid in 2-dimensional planar fracture is reasonable to a certain extent.