This article aims to reveal the control rules of pre-existing faults in the Cambrian-Ordovician carbonate rocks in the Manjiaer Sag of the Tarim Basin on the development density of structural fractures, and to provide geological basis for reservoir fracture prediction and oil and gas exploration and development. This study is based on cores, FMI imaging logging and regional geological data, and uses quantitative characterization of fracture surface density, curve fitting analysis and mechanical property comparison methods to systematically analyze the control effects of fault distance, length, dip angle and mechanical properties on fracture density. The results show that the fracture surface density decays exponentially as the fault distance increases, and the fitting coefficient of determination reaches 0.999 7. The density in the near-fault area can reach up to 2.81 m-1, and the far-end area is stable at 0.96~1.03 m-1. The fracture density increases linearly with the fault length and slowly increases with the fault inclination angle. The fitting coefficients of determination are 0.999 7 and 0.9160 7 respectively, both of which are secondary control factors. There are differences in the control effects of faults with different mechanical properties. The median fracture surface densities in tension, torsion, and compression fault control areas are 2.75 m-1, 2.00 m-1, and 1.30 m-1, respectively. Pre-existing faults control the spatial distribution of fracture density through stress redistribution and damage zone development, and the fault mechanical properties determine the control strength and attenuation mode. The research results can provide support for carbonate reservoir evaluation and exploration deployment in the study area.