We for the first time investigate comprehensive electrical characteristic fluctuation of gate-all-around silicon nanosheet metal-oxide-semiconductor field-effect transistors. The comprehensive fluctuation signifies the work function fluctuation, interface trap fluctuation, and random dopant fluctuation, simultaneously. Due to a complicated interaction of surface potential among fluctuation sources, simply considering each fluctuation source and calculating their total fluctuation will result in overestimated results, compared with the result of comprehensive fluctuation. 10.8% and 48.8% overestimations are observed for the threshold voltage and off-state current fluctuations. Notably, the characteristic fluctuation of the explored device is dominated by work function fluctuation because the channel surface potential is strongly altered by randomly localized work functions.