Abstract: Annular hydraulics is the key of slim hole drilling technology. The external and internal diameter ratio of slim hole drilling tool is relatively large and annular space is relatively small, resulting in large variation of annular pressure loss and rheological properties of drilling fluids. Many factors that are negligible in conventional drilling operation become very important in slim hole drilling. Herschel - Bulkley mode is used to describe the rheological properties of drilling fluids. An annular circulating pressure loss model of slim hole is built taking consideration of the influences of drill string decentralization and rotation on flow of drilling fluids in annular and drilling string. Comparison of model results with the conventional calculation results and measured data shows that this model has relatively high accuracy and can meet the requirements of field operation.