Abstract: A hardness separation system for steel ball based on preventive multi-frequency testing eddy current technology was presented. The composing,working principle,hardware and software design of the system were introduced. Taking industrial control PC as the core and worked with eight frequencies the system identified the hardness through overall evaluation of the measuring signal of each frequency. The experimental results and practical application in production proved that the system could be used for 100% automatic sorting to hardness of the steel balls due to its high sensibility and high reliability.