Abstract: Given the TIV anisotropy characteristics of low porosity/permeability formations, the accurate calculation of horizontal in-situ stress is essential for the perforation and fracturing designs of those reservoirs. In order to more accurately and effectively calculate the horizontal in-situ stress of a TIV formation, considering the low-pressure characteristics of such formation, the Bowers method was used to obtain the formation pore pressure. Then, the array acoustic logging data was used to obtain the anisotropic rock mechanical parameters (the vertical/ horizontal Young’s modulus of elasticity and Poisson's ratio) of TIV formation. Considering the influence of bedding plane occurrence on horizontal in-situ stress, the traditional Sn model was improved and a new calculation model for the horizontal in-situ stress of TIV formation was established. The horizontal in-situ stresses of the TIV formations in the Chang 6, Chang 7 and Chang 8 sections of the Heshui area of the Ordos Basin were calculated by this new model. The maximum relative errors between the calculated and the measured maximum/minimum horizontal in-situ stresses were 8.70% and 7.86%, respectively, which were smaller than those of in-situ stresses calculated by Sn model. The results showed that the horizontal in-situ stress calculated by this new model was more in line with the variation laws of the vertical and horizontal distributions of the actual in-situ stress, which could provide a more reliable reference for the hydraulic fracturing design of tight oil reservoirs.