Depended on Huangling — Yan'an expressway, directed against special topography, geology and hydrometeorology conditions in loess donga, subgrade drainage technique is studied comprehensively, which includes the followings:1. Based on the analysis of topography, geology, rain character and wreck cause of subgrade drainage, the paper gives commonly evaluation and fuzzy synthetically evaluation on design and construction of subgrade drainage in loess donga.2. According to hydrology and hydraulics theory, designed indexes such as : rinse and silt test, design storm frequency, safety depth of ditches and revising coefficient of the greatest grant velocity of open channel are analyzed and perfected.(1) Aimed at natural conditions in loess donga, the difference of ditch structure and the difficult degree of repair, proposed values of design storm frequency of ground drainage facility in loess donga is put forward, and proposed values for safety depth of all class roads and different sorts of ditches is presented; (2) A formula for revising coefficient on the greatest grant velocity of channel flow is presented, and the formula can calculate revising coefficient according to hydraulic radius and coefficient of roughness; (3 ) Based on the analysis of the coupling of rain intensity and convergence time of drainage, proposed values of parameter is given.3. Using IDF mathematic model and rainfall materials of Yan'an, this paper deduces rain intensity formula to be used to compare with others. On the base of overland flow theory, by one-dimension kinetic wave model, the formula for overland flow is deduced, which includes configuration and land feature in loess donga.4. Based on underground-water kinetics theory, the paper analyses existing formula for underground trench's depth, puts forward the formula for one or two underground trench's depth, which considers the influence of load depth and superelevation.5. The relation between the interval and depth of double underground trench is studied, and the formula of flux for double underground trench is brought forward. Based on the current research results, proceeded from descent curve of gradual change seepage, with underground-water theory, calculating method of designed flux for underground trench is improved, and the method is tested by field observation