The coiled tubing (CT) drilling technology in slim holes with many advantages has been unfortunately restricted due to tight space for fluids flowing, small flow rates, high flowback rates, huge loss in circulation. In order to solve the problem of overly large fluids flow resistance in the CT drilling, we calculated the flow resistance of drilling fluid in CT based upon the Herschel–Buckley Model and the features of this technology, and discussed the relationships between the fluids flow resistance and the other factors such as average flow rates of fluids, CT length (or well depth), CT water-hole diameter, tubing roller diameter, etc. The following findings were achieved. (1) On the tubing roller, CT internal flow resistance increases linearly with the drilling fluids flow rate and drilling fluids with small flow rates will decrease the flow resistance on the roller. (2) Both on the tubing roller and in the wellbore, CT drilling fluid flow resistance increases linearly with CT length but decreases linearly with CT internal water-hole diameter; therefore, large-diameter CTs will be better for drilling deep wells. (3) The tubing roller diameter has little effect on the CT fluids flow resistance, which increases with the fluids flow rate linearly; The CT wound on the roller is long enough to use multiple rollers so as to reduce the flow resistance of fluids in CT. In conclusion, this study provides a reference for controlling and reducing the CT fluids flow resistance and optimizing the relevant parameters, helpful to accelerate the popularization and application of this technology.