A photosensitive detector with a composite dielectric gate MOSFET structure comprising: n-type semiconductor regions located on the two sides of the top part of a p-type semiconductor substrate to form a source and a drain; a bottom dielectric layer, a photoelectron storage layer, a top dielectric layer, and a control gate stacked on the substrate in sequence; wherein said photoelectron storage layer is made from polysilicon, Si 3 N 4, InGaN, metal film or other electronic conductors or semiconductors, said control gate is made from polysilicon, metal or transparent conducting electrode; wherein said top dielectric layer that is connected to the control gate prevents the photoelectrons stored in the photoelectron storage layer from leaking into the control gate, under low gate voltage, said bottom dielectric layer connected to the p substrate effectively shields the channel between the source and the drain from the photoelectron storage layer, if the gate voltage or photon energy is high enough, the electrons in the channel can be injected into the photoelectron storage layer, and while photoelectrons are collected and injected, the source and the drain should be floating; wherein said bottom dielectric layer is made from silicon oxide, SiON or other materials with high-k dielectrics, said top dielectric layer is made from SiO 2 /Si 3 N 4 /SiO 2, SiO 2 /Al 2 O 3 /SiO 2, SiO 2, Al 2 O 3 or other materials with high-k dielectrics; wherein when the voltage difference between the gate and the substrate is high enough, the photoelectrons collected in the channel can tunnel into the photoelectron storage layer, and at least one of the substrate or the gate surface must be transparent or semitransparent for detecting the wavelength; wherein the detector is adapted for grounding the gate and the substrate, and apply a proper positive voltage on the drain, to regulate gate voltage to ensure that a MOSFET detector work in the linear region, characterised in that the detector is adapted for measuring and comparing the output drain current before and after exposure to light, to determine the optical signal strength according to the following formula: Δ I DS = μ n C ox W L ⋅ N FG q C T ⋅ V DS where Δ I DS is the detector drain current change before and after exposure to light, N FG is the number of photoelectrons in the photoelectron storage layer, C T is the total equivalent capacitance of the photoelectron storage layer, C ox is the gate oxide capacitance per unit area between photoelectron storage layer and the substrate, W and L are the detector channel width and length respectively, μ n is the electron mobility, V DS is the drain to source voltage, and the detector is adapted for resetting for applying the gate a negative voltage and grounding the substrate. n-type semiconductor regions located on the two sides of the top part of a p-type semiconductor substrate to form a source and a drain; a bottom dielectric layer, a photoelectron storage layer, a top dielectric layer, and a control gate stacked on the substrate in sequence; wherein said photoelectron storage layer is made from polysilicon, Si 3 N 4, InGaN, metal film or other electronic conductors or semiconductors, said control gate is made from polysilicon, metal or transparent conducting electrode; wherein said top dielectric layer that is connected to the control gate prevents the photoelectrons stored in the photoelectron storage layer from leaking into the control gate, under low gate voltage, said bottom dielectric layer connected to the p substrate effectively shields the channel between the source and the drain from the photoelectron storage layer, if the gate voltage or photon energy is high enough, the electrons in the channel can be injected into the photoelectron storage layer, and while photoelectrons are collected and injected, the source and the drain should be floating; wherein said bottom dielectric layer is made from silicon oxide, SiON or other materials with high-k dielectrics, said top dielectric layer is made from SiO 2 /Si 3 N 4 /SiO 2, SiO 2 /Al 2 O 3 /SiO 2, SiO 2, Al 2 O 3 or other materials with high-k dielectrics; wherein when the voltage difference between the gate and the substrate is high enough, the photoelectrons collected in the channel can tunnel into the photoelectron storage layer, and at least one of the substrate or the gate surface must be transparent or semitransparent for detecting the wavelength; wherein the detector is adapted for grounding the gate and the substrate, and apply a proper positive voltage on the drain, to regulate gate voltage to ensure that a MOSFET detector work in the linear region,