Abstract: Alternating current field measurement (ACFM) technology has been widely used in ocean engineering for detection and evaluation of cracks on structures. Conventional testing method is easy to introduce miscalculation and is hardly to achieve real-time determination and evaluation of cracks based on characteristic signals or butterfly-plot using ACFM probe. A new judgment method is presented based on ACFM system with sensor array. The energy spectrum of B_x and the derivative of B_z phase are combined together as threshold to judge cracks in real-time. The dangerous grade of cracks is evaluated based on the threshold. The crack test and evaluation experiments are carried out. The results show that the energy spectrum of B_x can be set as the threshold of crack length. The B_z phase reverses greatly when the crack is present. The cracks can be confirmed by the threshold real-time determination method. The dangerous grade of cracks can be evaluated using the threshold parameters. The cracks can be evaluated and located by the ACFM testing system based on the energy spectrum and phase derivative threshold.