Abstract: To address the challenges of signal attenuation, side-lobe acquisition difficulty, and position dilution of precision (PDOP) degradation when GNSS signals serve spacecraft in cislunar space, this study investigates the feasibility of utilizing GNSS weak signals for positioning, navigation, and timing (PNT) of lunar-orbiting spacecraft. Antenna characteristic calibration is conducted based on in-flight weak signal experiment data from the distant retrograde orbit (DRO) satellite DRO-B of the Chinese Academy of Sciences. A multi-GNSS simulation platform is developed, and a hardware-in-the-loop system is established using the DRO-B backup receiver and the Spirent GSS9000 simulator. Receiver performance under different tracking sensitivities is evaluated, followed by geometric positioning and dynamic orbit determination analyses. Results indicate that with the receiver sensitivity of −176 dBW, GNSS weak signals could effectively support spacecraft positioning and orbit determination during the Earth-Moon transfer phase within 130 000 km, with a geometric positioning error at 100 m level. During the lunar orbiting phase, the receiver sensitivity must be further enhanced to −190 dBW to meet requirements, achieving a geometric positioning accuracy of 1.67−4.04 km (three-axis) and a orbit determination accuracy of 0.02−0.06 km (three-axis) after convergence. Spaceborne GNSS could be an effective supplement to ground-based orbit determination techniques. Stable positioning and timing using multi-GNSS near the Moon is applicable in the future by further improving acquisition and tracking sensitivity and integrating multi-source measurements.