Abstract: Standard penetration depth is influenced by various factors, and the effective detection depth evaluated based on standard penetration depth may greatly deviate from the actual one, thus influencing the detection of deep defects. In this paper, the effective penetration depth was theoretically calculated and measured by experiments; and the comparison reveals that the measured value is much smaller than the estimated one. The causes of such deviation were analyzed to provide a reference for detection parameters optimization and data analysis in practical testing.