Tillage depth plays an important role in crop growth and should be well managed during cultivation. Herein, an adaptable tillage depth monitoring system, which is provided with a surface-fitting swing arm and an optical encoder to measure the rotation of the swing arm, has been designed and developed. In the paper, the processes of uphill and downhill, which have been divided into 5 stages, are analysed respectively. Moreover, a LabVIEW program that can be employed to adjust the depth measurement according to the pitch angle between the implement and the ground, as well as the inclined angle of the ground, has been established to achieve adaptable measurement on different terrain morphologies. The system uses different models to allow for the variation of the angle and displacement in the process of tractor-implement combination going through the slope, and thus can achieve adaptable measurement of tillage depth. Besides, the tillage depth monitoring system can not only monitor tillage depth in real time, but also display a graphical trace of tillage history. Field experiments have been conducted to evaluate the system's performance, and have demonstrated good accuracy on both regular surface and sloped surface, showing maximal absolute errors of 11.3 mm and −12.8 mm, as well as maximum relative errors of 7.40% and 8.53% for the field experiments respectively. Hence, such a measuring system holds good potential for its application to the current tillage depth monitoring, particularly in the case of covered ground as in conservation farming.