The conversion between Grid DEMs and TINs is of critical significance in digital terrain analysis which is a key work in the watershed planning of the loess plateau region. Taking Jiuyuan watershed in Suide county of northern Shaanxi province as a test area and 1 :10 000 Grid DEM as a criterion, applying overlay comparative laws and mathematical statistics as basic research methodology, a research on the accuracy of conversion between Grid DEMs and TINs is made. The experiment shows that the conversion accuracy between Grid DEMs and TINs affected by conversion tolerance of TINs directly. The accuracy of Grid DEMs on describing terrain variables (i.e. elevation, slope gradient, channel network etc) decreases with conversion tolerance of TINs increasing, compared to Grid DEM of 1: 10 000. Five meters should be a rational conversion tolerance in the loess hill and gully area.