With the advancement of ultra-deep well drilling and dry hot rock energy development, the demand for measurement-while-drilling (MWD) systems has grown increasingly critical. The electronic MWD system is difficult to complete the measurement in the downhole environment above 200 °C. In this study, a novel fully mechanical well deviation measurement system is designed, utilizing a bias structure to sense inclination and translate it into flow-related data. The system leverages the turbineʼs flow sensitivity to determine the well deviation. A mathematical model for well deviation sensing and signal coding was established, and a signal pulse generator capable of reflecting well deviation changes was also built. The fluid simulation of the pulse generator was carried out by using computational fluid dynamics (CFD) simulation software. The stable output state of the pulse generator under different flow rates was calculated. The feasibility of the fully mechanical well deviation measurement system was verified through theoretical calculation and simulation analysis. An experimental system for mechanical well deviation measurement was established, and the performance of mechanical well deviation sensing and measurement was analyzed. A downhole experiment was carried out to verify the reliability of this mechanical well deviation measurement system in achieving mechanical well deviation measurement.