Abstract: In order to classify the quantitative characterization method of structural fracture length in low-permeability reservoirs, the Cambrian Longwangmiao Formation of Moxi-Gaoshiti areas in the Sichuan Basin is taken an example in this study. The relationship between structural fracture lengths of low-permeability reservoirs was derived from the perspective of structural stress field by the means of core fracture statistics and rock mechanical experiment. Quantitative relationships between fracture length and fracture volume density, strain energy density and rock mass stress state were established. Results showed a negative exponential power relationship between fracture length and number, a proportional linear relationship between fracture volume density and strain energy density, and a negative exponential power relationship between fracture length and volume density. The derived fracture length formula was applied to the Longwangmiao Formation in Moxi-Gaoshiti areas. Numerical simulation results showed that fracture density generally ranges from 1-5 m²/m³, and the highest value is 9 m²/m³, mainly distributed in fault and its surrounding areas. Fracture length is mainly between 1-20 m. The fractures in fault and surrounding areas are dense and short, usually less than 3 m.