Thin-film gated silicon-on-insulator (SOI) lateral PIN photodetector is a novel photodetector, based on standard SOI technology and CMOS process, combining the conventional bipolar and field effect photosensitive device structure. The basic structure of this novel photodetector is described and the operation principle is analyzed. Applying physical equations of semiconductor device, the gate voltage and photocurrent models can be built. Two-dimensional numerical simulations are performed in SILVACO software. In the middle-short wavelength operation period, the output photocurrent increases with gate voltage, with the obvious gated-control characteristics. Under fully depleted condition, the internal quantum efficiency can yield over 96%, even near 100% for the varied wavelengths(400nm, 450nm, 530nm, 600nm). For short wavelengths(280nm, 350nm), the internal quantum efficiency is relatively lower, the maximum is approximately 80%. And the dark current of this photodetector is low, leading to a high ratio of more than 10⁶ between illuminated to dark current, achieving high sensitivity.