To consider the needs for drilling and the principles of carry and suspend formation cuttings, the permitted yield point control boundary model of drilling fluid was derived using solid-liquid dynamics under certain given assumptions. Corresponding calculation software was developed in c#. The results show that: Any one of the drilling fluid density, pump discharge, plastic viscosity and drilling fluid pump cylinder bore increases would result in the permitted yield point control boundary which turned out to be narrow. Drilling fluid pump discharge is the main cause of the decrease in the upper range of the permitted yield point control boundary. While drilling fluid density is the main cause of the decrease in the lower range. Application had shown the practicality and accuracy of this model in Tarim oil field KS8 well.