Abstract: Unlike the conventional fracturing operation, the fracturing fluid not only flows in straight section of downhole coiled tubing, but also flows into the part of coiled tubing which is wound on the reel. The flow in spiral section is very complex, and the calculation results from existing model is somewhat different from the actual engineering practice, so a general relation was given to the friction factor in the spiral section of the coiled tubing based on principle of flow mechanics and in combination with friction factor equation of the straight section and the geometric characteristics of the spiral section; finally a complete calculation model for internal pressure drop during fracturing operation in coiled tubing was built with theoretical derivation. Analysis was conducted of the effect law of parameters like coiled tubing diameter, drum diameter, displacement, viscosity and flow index on internal pressure drop. The result shows that the accuracy of the calculations by this model is high; under the same conditions, the pressure drop in spiral section is always greater than that in straight section; the diameter of coiled tubing poses the greatest impact on pressure drop; when the diameter is increased by 1 time, the pressure drop reduces by 13 times, while the drum’s diameter has the smallest impact on pressure drop, so no matter what size of the drum is, the pressure drop almost remains the same.