The slender truss boom is a key working component of the crane with truss boom, and unloading rebound impact is an important working condition that threatens the safety of slender truss boom crane. To address the dynamic behavior of slender truss booms under unloading impact, this paper used rigid flexible coupling multi-body simulation method and crane unloading impact experimental method to explore the variation law of dynamic stress under unloading rebound conditions of the truss boom, and the unloading impact dynamic load coefficient was calculated based on the dynamic stress of the truss boom. A refined simulation model of a boom tower crane equipped with slender truss booms was established using a rigid flexible coupling method, which includes load model and structural dynamic characteristic model. It analyzed the dynamic stress changes caused by the rebound vibration of the truss boom, and the distribution law of peak dynamic stress during unloading rebound of truss booms was discovered. According to the lifting performance table of cranes with different boom lengths, the relationship between the elevation stress relationship curve and the lifting performance curve was studied, and the sudden unloading condition of the crane corresponding to the maximum stress in the middle of the boom occurred was found. Based on the simulated results of crane unloading impact, a crane unloading impact experiment method based on simulation prediction was established. The sudden unloading impact experiment of the series of boom tower cranes was carried out. The error between the experimental and simulated values of the truss boom dynamic stress is less than 13%, proving that refined model simulation is an effective tool for solving the unloading impact dynamic response of truss boom. Through model simulation, the unloading impact dynamic load coefficient of the slender truss boom under critical situations was further predicted. It finds that there are defects in the relevant regulations on unloading impact dynamic load coefficient in the current crane design specifications. The impact of the truss boom slenderness ratio on the unloading impact dynamic load coefficient was explored, providing a basis for the optimization design of key crane structures.