The invention discloses an on-line monitoring device and method for a moisture content of a deep rock and soil mass in an open-pit dump. The monitoring device comprises a plurality of moisture sensors, a solar panel, and a data acquisition module. The plurality of moisture sensors are disposed in geotechnical boreholes extending vertically from the stepped platform to a potential slip surface below a dump, and use the potential slip surface of the dump as a symmetry axis to be uniformly distributed along an axial direction of the geotechnical boreholes. The plurality of moisture sensors are connected in parallel with the solar panel through a power supply line, and are connected in parallel with the data acquisition module through a data acquisition line. The method ensures that the moisture sensors do not interfere with each other, ensures that the data acquisition module collects data of each moisture sensor in real time, and ensures the accuracy of the monitoring through a pluralityof parallel moisture sensors. In addition, the solar panel is used for supplying power to the moisture sensor, thereby meeting the requirements of different dumps, and facilitating to arrange the device according to the dump. The invention combines wireless transmission to improve monitoring efficiency.