Drilling by high-pressure liquid jet, radial jet drilling (RJD) is a cost-effective technology to restimulate the production of old wells and the development of unconventional reservoirs. However, due to the unique process of 90 deg turning in the casing, hardly any traditional tool can be applied in the RJD trajectory measurement. Even minitools have been developed in the last few years; the increasing cost and unpredictable failure risk of lateral re-entrance are still non-negligible as the current design, much less than the huge measuring errors. In this paper, a new tool was proposed. Based on a special-shaped circuit board and separation of the supporting section, a fluid passageway was reserved in the measuring section to realize the measurement while or after the jet drilling without extra trips. It reduces the cost and failure risk of lateral re-entrance. Ample space in the supporting section was also provided, which meets the long-time operation requirements and establishes the base for real-time communication and trajectory control. Based on noise analyses, random walk is the main source of system noise in short-time measurements, and the effective measuring frequency is mostly in the range of 0–5 Hz. Therefore, autoregressive moving average (ARMA) models, Kalman filter, and low-pass filter were established for denoising. It allows us to analyze the data feature in more detail and extract the valid measuring data with improved accuracy. Considering performance tests result, the average errors of reckoned parameters were improved to less than 15%.