Abstract: The current method, which relies on the peak impact force reduction rate, tends to overestimate the protective performance of attached anti-vessel-collision facilities, thus compromising operational safety. An improved calculation method is therefore proposed. The procedure began with an analysis of the time-history load frequency components of the impact force. And the optimal impact load characteristic of the protection facility was investigated. The slope of the impulse time-history curve was established as the critical parameter governing the protective performance, which subsequently allowed for the determination of the lower-bound protection limit and the upper-bound stiffness limit for the attached protection facility. Subsequently, the filtering method, the cumulative impulse method, and the impulse-slope method were proposed. A preliminary analysis of their accuracy established the superiority of the impulse-slope method in terms of both precise results and broad applicability. Finally, low-velocity impact tests on five scaled models of anti-collision facilities and full-scale high-energy impact simulations of 42 types of protective devices were conducted to validate the accuracy of the improved method. The results indicate that when an attached protection facility is classified as low-efficiency, the existing performance calculation method can significantly overestimate its capability. The maximum difference reaches 500%. Therefore, this overestimation can lead to inadequate protection for the bridge pier, resulting in an unsafe bias in the assessment. When evaluating facilities with medium protective efficiency, the conventional method can lead to significant deviations in the results; nevertheless, it still meets the minimum performance criteria stipulated by design codes. For facilities with high protective efficiency, the peak force method, filtering method, and slope method all yield reliable assessments of their performance, with each showing a force reduction rate of over 40%. In this case, the protection facility operates at optimal protective performance, providing the highest degree of protection to the bridge pier. The improved calculation method proposed in this study enables a more precise evaluation of the protective performance of various types of attached pier protection facilities. The findings can serve as a technical basis for future revisions of relevant design code provisions.