This research presents the implementation of the flexible dynamics model of a mobile crane structure in spatial coordinates that could simulate the motion using a short calculation time. The mobile crane model is consisted of the components with the assumptions of the rigid bodies, a flexible body, a point mass. The model of the flexible body is considered with the floating frame of reference formulation. In this work, the problems of the numerical stiffness from high natural frequencies, and the cost of calculating the time-variate inverse matrix of inertia are focused. The system-level model reduction of the adaptive modal integration (AMI) is applied. By using AMI, the model is discretized and linearized throughout its movable configuration, then the dynamics model is presented in form of modal coordinates systems. The lowest 14 out of 26 mode shapes are selected to represent the flexible motion of the model. The mode shapes, and the inertia matrix and its inverse are precalculated and interpolated during the simulation. The reduced model is constructed such that it could be calculated with the arbitrary pattern of input motion. The numerical simulation is performed to compare the reduced model and the nonlinear model. The reduced model able to illustrate an accurate result under shorter calculation time. The truncation of high frequencies modes encouraged the larger integration step while the interpolation reduced the cost of calculating the inverse matrix are resulted in short calculation time.