Practical boom crane systems are often used for land and marine transportation, where the accurate positioning and posture adjustments of distributed-mass payloads are crucial during lifting, placement, and assembly. However, common single-cable lifting systems are difficult to meet actual assembly requirements. Moreover, as a kind of typical underactuated system, unactuated payloads may easily have large swing angles to degrade operating performance. To this end, the paper establishes a nonlinear dynamic model for boom crane systems equipped with variable-length dual cables and spreaders with the consideration of posture adjustments, comprehensively reflecting the nonlinear coupling relationships among both actuated and unactuated variables. By combining the dynamic characteristics of the spreader and the distributed-mass payload, the payload posture is changed by adjusting the length of the spreader, and the motions related to the two payload swing angles are regarded as to-be-controlled variables. The proposed model lays the foundation for the controller design of subsequent lifting, placement, and assembly. To the best of our knowledge, this is the first nonlinear dynamic model established for boom crane systems with variable-length dual cables and distributed-mass payloads (without any linearization), which ensures accurate dynamic characteristics even when the payload swing angles significantly deviate from the equilibrium point.