Abstract Radial jet drilling (RJD) is an unconventional drilling technology to drill multiple radial laterals by using a high-pressure liquid jet. It is a cost-effective alternative to bypass damage zones near the wellbore, restimulate the production of old wells and develop the unconventional reservoirs. However, due to the structure of the deflector and the diameter of the radial laterals, traditional well-trajectory measuring tools cannot be applied in the RJD wells. It hinders the conduct of some significant operations. The unknown trajectory is a crucial limitation to further development and field application of the RJD technology. In this paper, except an introduction of RJD technology, a measuring system and a mini-tool were proposed for the attitude measurement and motion state recognition. Based on the navigation theory, a reckoning method of the trajectory was established. After that, an experiment study was carried out to test the performance of the measuring tool and the reckoning method. As results of the experiments, the average errors of the measured lateral length, inclination and azimuth are 8.12%, 5.10% and 5.10% respectively.