In order to obtain the flow resistance characteristics of the primary side of the Helical coiled Once-Through Steam Generator (HOTSG) helical pipe section, a three-dimensional simulation analysis models of the primary side of the helical pipe section is established by using the computational fluid dynamics software STAR-CCM+, this model is the same scale as the analysis prototype, which has three layers helical pipe in radial direction and about four meters in axial direction. The resistance coefficients of the primary side of the helical pipe section under different inlet flow rates are calculated with this model. The calculation results are compared with the hydraulic test resistance characteristic results of the primary side of the same HOTSG prototype. It is shown that the model can be very good to forecast the resistance characteristic of prototype, the resistance coefficients under different inlet flow rates are very close to test results. On this basis, in order to study the scale of analysis model to reduce the simulation costs, some different reduced models are studied, including three aspects: (a) 1/10 and 3/10 of the prototype dimension in axial height direction, (b) 60° and 180° angle cutting sector of the prototype circumferential dimension, (c) different layer numbers of helical pipe in radial direction. The deviation of calculated results of different scale models on resistance characteristics are obtained, which can provide reference for the analysis of resistance characteristics of fine structure of helical pipe section with larger scale.