Disclosed are an online monitoring system for a crack on a hoist spindle and an operation method thereof. The system comprises: a rope power part, a crack detection part, a wireless transmission part, and a computer. The rope power part comprises two traction ropes, two guide wheels, two stepper motors, and two stepper motor drivers. The crack detection part comprises a spiral tube guide rail, a sliding body, and an ultrasonic generator. The wireless transmission part comprises three zigbee wireless sensing modules. The zigbee wireless sensing modules receive instructions from the computer and transmit the instructions to the stepper motor drivers to control the motors to rotate. The stepper motors drive the guide wheels to rotate to realize the winding of the ropes, so as to pull the sliding body to slide on the spiral tube guide rail. The ultrasonic generator clamped on the sliding body monitors the rotating spindle along the spiral tube guide rail. The zigbee wireless sensing modules transmit the detected data to the computer in real time. The present invention can effectively monitor a hoist spindle in time before a failure occurs, thereby avoiding safety accidents.