The International Space Station continues to be a productive space research laboratory after decades of operations, and it has proven to be a variable resource for observing spacecraft floating potential variations related to high-voltage solar array operations in ionospheric plasma. In recent years, a new category of solar-array-induced charging event has been observed and remains not well understood so far. The charging transients appear when the shunted solar arrays are unshunted in full sunlight , characterized by sharp rise in spacecraft floating potential on a timescale of milliseconds followed by a fall on a timescale of tens of millisecond. This kind of sunlight unshunt transients are clearly distinguished from the rapid charging events due to exit of eclipse, although both are related to high-voltage solar array operations. In this paper, a physical model is built for the sunlight unshunt transient charging. The model results agree well with the flight observations both in pulse timescales and peak amplitudes. It indicates that the primary mechanism underlying the characteristics of the transient charging lies in two aspects, the rapid panel voltage switch-on in full sunlight and the instantaneous blocking effect caused by the cover glass neutralization. In addition, the model also predicts the tendency of the transient charging varying with ionospheric plasma and other space weather conditions.