Conventional infrared spectral imaging detection can provide large amount spectrum information and has extensive applications, but it cannot provide spectrum tunablility with compact volume. A novel FET detector structure with electrically tunable plasmon absorption was proposed. Through electromagnetic simulation of infrared absorption variation with the structure parameters, more than 90% absorption is reached within specified spectral band under optimized condition for this kind of detector. When gate voltage is changed, the carrier concentration and complex refractive index vary simultaneously, which lead to obvious characteristics of tunable absorption. Based on this FET structure, uncooled infrared detector will have the ability of spectral imaging on one single chip, which opens a new route to miniaturization of spectral imaging systems.