The conventional hydraulic fracturing (HF) method and the hydraulic tests on pre-existing fractures (HTPF) are limited greatly in the in situ stress measurements of inclined boreholes, especially in three-dimensional stress measurements. Moreover, the HTPF method requires a large number of pre-existing fractures to obtain reasonable results. By optimizing the assumption of shear stresses on the pre-existing fracture surfaces and using the Monte Carlo algorithm, the three-dimensional stress tensor inversion in the inclined borehole was realized by combining the conventional HF tests and the HTPF results. The reliability and precision of the proposed method were verified by three-dimensional stress measurements of an inclined borehole in a pumped storage power station, in Tianjin province. The results show that the method proposed in this paper can obtain the three-dimensional stress tensors of the inclined borehole only by selecting one or more conventional HF tests in the intact rock mass and HTPF results, respectively. Compared with the HTPF method that only adopts pre-existing fractures in stress calculations, the proposed method greatly reduces the requirement of the number of pre-existing fractures using the Monte Carlo algorithm, combined with the HF method. The inversion results have little dispersion and high stability, which are also consistent with the regional stresses.