Abstract: The theory of pressure drop in annulus and its affecting factors were described in this paper. The different equations of annular pressure drop related with these above mentioned factors were deduced. Some flow patterns as Coutte Flow, Dylaw Vertex and Dylaw Value were described. Different types of drill string movement, influences of drilling fluid rheology and viscosity on annulus pressure drop were interpreted. The paper demonstrated that substantial changes of pressure drop, drilling fluid rheology and drilling fluid flow pattern occur because of the greater ratio of Do/Di of the drill string and minor radial width of the annulus in slim well. Some factors which can be ignored in ordinary well drilling appear particularly important in slim well drilling.