Within the technical field of detecting a mining-induced failure range of rock mass, and particularly relating to a graded-pressure drop type detection system and detection method of borehole cracks, a detection system is provided including a test probe, a drilling rig, a drill pipe and a control operation platform. The test probe includes a plugging device and a pressure conversion assembly. The pressure conversion assembly is connected with the plugging device at the rear end through a connected pipe and includes a first stage pressure converting device and a second stage pressure converting device. The detection system simplifies the existing operating system, avoids the problem of drill pipes winding in drill hole, realizes plugging process and detection process with the same external water source, and ensures that the plugging process and the observation process operate under respective predetermined pressures.