The paper presents the statical research tests of rod bolt made of plastic with a length of 5.5 m, which were performed in a modern laboratory test facility at the Department of Underground Mining of the University of Science and Technology. Innovative The Self-excited Acoustic System (SAS) used to measure stress changes in the bolt support was characterized. The system can be used for the non-destructive evaluation of the strain of the bolt around the excavations as well as in tunnels. The aim of the study was to compare the re-sults recorded by two different measuring systems, thanks to which it will be possible to assess the load of long bolt support by means of the non-destructive method. The speed and simplicity of measurement, access to the sensors, accuracy of measurement and reading should be kept in mind in determining the load of rock bolt support . In addition, the possibility of damage to the sensor as a re-sult of technological or natural hazards should also be taken into account. In economic conditions, the „technical - balance laws of production”, which ex-cludes the use of load sensors on each bolt must be preserved. The use of indi-vidual load sensors of rock bolt support for the boundary state, allows appro-priate protection actions of the mining crew against sudden loss of excavation stability to be taken. The paper presents two basic effects used in the ultrasonic measurement sys-tem. The first result was the existence of stable limit cycle oscillations for posi-tive feedback. This effect is called the self-excited effect. The second effect is called the elasto-acoustic effect. It means that with the change of elastic stress-es in the material bring the change of the speed of propagation of the wave. In this connection, the propagation time between measuring heads is also changed. This effect manifests itself in the change in the oscillation frequency of the self-excited system. For this reason, by measuring the frequency of self-excited oscillation, it is possible to indirectly determine the level of effort of the tested material.