Abstract: Detection of defects extended from the surface to the axis of a spindle at the part of assembly area was performed to meet the requirements of monitoring the spindle quality in the running of wind turbines. An on-line monitoring system for spindle quality were designed. The system contains ultrasound transmitting, receiving circuits, wireless transceiver circuit, multi-channel selection circuit, and so on. The circular array elements were installed on the lateral surface, and the ultrasonic pulse reflection principle was used in the system. To verify the system effect, the detection of the spindle specimen and its artificial defects were carried out. Results show that the experiment is in good coincidence with theory, which achieve the requirement of defeats detection in the assembly area. This system not only solves the actual problem of spindle quality monitoring during its work, but also provides a set of implementations to remote online quality monitoring.