The utility model discloses a parasitic ultrasonic automatic detection system, comprising a computer, an ultrasonic probe device, an ultrasonic card and a numerical control machine, wherein the ultrasonic probe device is in communication connection with the ultrasonic card, the ultrasonic card is in communication connection with the computer via a PCI (Peripheral Component Interconnect) bus, an output end of the computer is connected to a control end of the numerical control machine via a RS232 serial port, the ultrasonic probe device comprises a probe bracket and an ultrasonic probe, the probe bracket is clamped at a tool clamping position of the numerical control machine, and the ultrasonic probe is connected to the ultrasonic probe. The parasitic ultrasonic automatic detection system has the following technical effects that: the ultrasonic automatic detection function can be realized by using a normal multi-axis numerical control machine, without designing and manufacturing a special ultrasonic detection test bed; and the development cycle is short, and cost is low.