The invention relates to an ultrasonic detection instrument for steel pipes, steel plates and other metal products, in particular to a distributed multi-channel ultrasonic flaw-inspecting system. The system consists of a plurality of ultrasonic modules, wherein each ultrasonic module is connected with 2 to 64 ultrasonic probes; the ultrasonic modules are connected with a host computer through an Ethernet switch; the host computer is connected with a display, a printer and an alarm device respectively; a positioning label-spraying module is connected with one of the ultrasonic modules; and thepositioning label-spraying module is connected with an external positioning label-spraying device. Compared with the prior similar products, the system has the characteristics of realizing function modularization under the circumstance that the number of channels is up to hundreds, along with fast flaw-inspecting detection, accurate defect discrimination, strong on-site anti-interference ability,free-combination capability, simple line connection and reliable function, and can realize on-site 365-day uninterrupted operation and real-time detection.