The work aims to study the working stability and the flow field structure of high-pressure compressors affected by fall-off of seal coating. The 12th-stage rotor of a high-pressure compressor was taken as the research object, and its single passage grid was generated so as to investigate the flow characteristics by numerical simulation under the taking-off condition and different fall-off thickness with Spalart-Allmaras one-equation turbulence model. As the seal coating fell off, the efficiency and the total pressure ratio decreased, and the stall margin declined exponentially in particular. The compressor reached the stall margin at 4τ fall-off thickness under the working flow rate. Under the working flow rate, the fall-off of seal coating led to a moving forward of the tip leakage vortex/incoming flow interface and the position that the secondary leakage flow entered the leakage vortex. Meanwhile, the backflow and repeated streamlines leakage in front of the blade tip could be observed. After the fall-off thickness was greater than 3τ, the leakage vortex core in the rear tip passage expanded violently, and a remarkable low-velocity region appeared. The unsteady simulation showed that when the fall-off thickness increased to 4τ, the tip leakage flow exhibited significant periodicity and self-induced unsteadiness, accompanied by instantaneous vortex breakdown phenomenon. Therefore, the fall-off of seal coating can dramatically degrade the working stability of the high-pressure compressor, and the main influencing factors are complicated flow and low-velocity region produced by the interaction of tip leakage vortex, secondary leakage flow and the main flow, which presents the unsteadiness of the flow field in the tip area.