Abstract: According to the monotone function relations laws between the metamorphic quality of piston alloy ZL108 and the material conductivity, eddy current testing was used to test the metamorphic quality. The research was proceeded by applying phase shifters, mixing, filtering technology to separate the resistance and inductance component from the signal tested by eddy current testing sensor. The circuit was designed to mix the sensor signal to power signal, so as to separate the impedance component; and also mix the sensor signal to π/2 phase shifter signal, so as to separate inductance component. Finally it was determined to use the resistance component to get the metamorphic quality information, thus eliminating the impact of inductance components and improved the measuring accuracy.