A strategy for enhancing the inspection precision of cracks in a thick structure is proposed. The key problem of inspection of cracks in a thick structure is the penetration depth of eddy currents. Using lower frequency to drive the probe can overcome it, but too low frequency will cause too small signal to noise ratio and inspection sensitivity. To solve these problems, effects of frequency, inspection mode, and probe type on eddy current testing signals were studied. At last, an effective strategy for inspection of cracks in a thick structure was proposed.