With the continuous advancement of offshore development in the South China Sea， the installation of large-scale marine structures poses increasingly higher demands on deep-water capabilities. Globally， the fleet of subsea construction vessels equipped with cranes exceeding 400 tons is extremely limited， and their charter rates are extremely high. Therefore， maximizing the utilization of existing resources to meet the installation needs of large deep-water structures has become an urgent issue that requires immediate resolution. The conventional lifting curve method， which requires the maximum dynamic load to remain below the safe working load， significantly underutilizes the crane’s capacity. This study begins with the design method of lifting curves to clarify the crane’s actual lifting capacity. By precisely capturing the relative motion among the barge， construction vessel， and crane， the inboard lifting curve can be applied during barge lifting operations， increasing the lifting capacity by nearly 30%. Based on this theoretical framework， we calculate the lifting load. Subsequently， by integrating the crane’s pre-alarm and alarm states to define three unity check （UC） values， a comprehensive assessment of the crane’s capacity is conducted. This study establishes a mature algorithm that has been validated in actual engineering projects. To better translate the theoretical algorithm into engineering practice， this study further develops an efficient software tool for lifting capacity assessment， which provides significant guidance for assessing the installation feasibility of large deep-water structures.