Abstract： Abstract：The theory of in-situ stress determination based on hydraulic fracturing tests on preexisting fractures(HTPF) is presented. The similarities and differences between the HTPF and the classical hydraulic fracturing(HF) technique are discussed. The inversion method for 3D in-situ stress field considering stress gradient along the vertical borehole axis is also introduced. In the course of searching for the principal stresses of stress field in a liquefied petroleum gas underground storage project，the regional stress field is supposed to be continuous only in a small section near the cavity area and the referential point is also chosen to be located in this area. Compared with previous studies that the regional stress field is assumed to be continuous in the whole depth of the borehole and the referential point is chosen to be at ground surface，the result by the proposed method is more reasonable. The complete stress tensor of regional stress field is obtained based on the genetic algorithm；and the result is satisfactory.
