Deep-level trapping effect is one of the most critical issues that restrict the performance improvement of GaN-based microwave power devices. It is of very importance for material growth and device development to study the trapping behavior in the device. In the past decades, there have been made a lot of efforts to characterize and investigate the deep-level trapping phenomena. However, most of the previous researches focused on the large-scale devices. For pursuing higher frequency, the devices need to be scaled down. Consequently, it becomes more difficult to characterize the deep-level traps in small-scale GaN-based devices, since none of the traditional characterization techniques such as capacitance-voltage (C-V) measurement and capacitance deep-level transient spectroscopy (C-DLTS) are applicable to small devices. Pulsed I-V measurement and transient simulation are useful techniques for analyzing trapping effects in AlGaN/GaN high electron mobility transitors (HEMTs). In this work, AlGaN/GaN metal-oxide-semiconductor HEMTs (MOSHEMTs) with very short gate length (Lg=80 nm) are fabricated. Based on the pulsed I-V measurement and two-dimensional transient simulation, the influence of deep-level trap on the dynamic characteristic of short-gate AlGaN/GaN MOSHEMT is investigated. First, the pulsed I-V characteristics of AlGaN/GaN MOSHEMT with different quiescent bias voltages are studied. In addition, the current collapse induced by the trapping effect is extracted as a function of the quiescent bias voltage. Furthermore, the transient current of AlGaN/GaN MOSHEMT is simulated with the calibrated model, and the simulation exhibits a similar result to the measurement. Moreover, the physical mechanism of trapping effect in the device is analyzed based on the experimental data and simulation results. It is shown that the current collapse of AlGaN/GaN MOSHEMT varies non-monotonically with the increase of the gate quiescent bias voltage, which results from the combination effect of the gate leakage injection-related and hot electron injection-related mechanism. In the off state, the current collapse is mainly induced by the traps below the gate, which is dominated by the gate leakage injection mechanism, leading to the decrease of current collapse with the increase of the gate bias voltage. In the on state, the hot electron injection mechanism becomes the dominant factor for trapping effect in the drain access region, resulting in the increase of current collapse. The results in this work indicate that the trap-induced current collapse can be further suppressed by improving the quality of gate dielectric to minimize the gate reverse leakage and by reducing the trap density in the epitaxial layer.