In this paper, the ultrafast transient nonlinear optical response and broadband dynamics mechanisms of carriers based on Fe defects in Fe-doped gallium nitride (GaN∶Fe) crystals were investigated with multi-dimensional pump-probe techniques. The results of the phase object (PO) pump-probe experiment show that the refraction dynamics curve of the carriers exhibits an obvious recovery compared with their absorption curve, and the recovery is due to the broadband absorption of Fe defect states according to the ultrafast transient absorption spectroscopy experiment. Furthermore, both the transient absorption response and the carrier trapping rate can be tuned over a wide range by the Fe content, and the absorption amplitude enlarges and the lifetime of trapped carriers shortens as the Fe content increases. On the basis of the transient optical nonlinearity results, this paper proposes an excitation and trapping model based on the different charge states of the Fe defects. The carrier trapping mechanisms in GaN∶Fe and the important parameters of Fe defect-related trapping rate and optical absorption cross-section are obtained by global analysis and rate equations. The tunable carrier lifetime and ultra-broadband absorption spectra in GaN∶Fe are of great significance for the design and development of optoelectronic devices, such as optical switches, optical limiters, and optoelectronic detectors.