Cable joints represent the most vulnerable component in power cable systems. Detecting partial discharge (PD) in cable joints is the most effective method for preventing cable system failures. However, the unique structure of cable joints makes it extremely challenging to apply conventional sensors for PD detection within the insulation of cable joints. Building upon the flexible sensor unit, this paper proposes a PD signal sensing method designed for deployment in the confined spaces between the outer shielding layer and the protective sheath of the joint. Firstly, finite element simulation software was employed to design and analyze the effective operating frequency band of the flexible sensor unit within the cross-linked polyethylene (XLPE) medium. Through adjustments and testing of its radiation pattern, the operational bandwidth was optimized to cover typical partial discharge signals generated during cable joint failures, thereby ensuring that its detection capability is maintained when the sensor is in a bent state. Secondly, the propagation characteristics of high-frequency electromagnetic radiation from PD signals through the elongated multi-layer dielectric space of a cable joint were calculated to determine the optimal installation position for the flexible sensor unit. This analysis confirmed that this position for the flexible sensor unit to be near the stress cone of the cable joint. Finally, an experimental platform for cable joint PD was established to analyze the performance of the flexible sensor unit. Simulation and experimental results demonstrate that the flexible sensor unit operates effectively within a frequency band of 50 to 640 MHz in the XLPE medium, with a bending radius of 6~10 cm without compromising detection performance. The amplitude of high-frequency electromagnetic radiation from PD signals is significantly enhanced near the stress cone of the cable joint. The flexible sensor unit can effectively detect composite defects located 12~24 cm from the PD source within the cable joint. The flexible sensor unit exhibits satisfactory detection performance in practical applications on 35 kV cables. It enables successful detection of PD signals within the confined spaces of cable joints.