Abstract: In terahertz imaging focal plane system, as the key device of THz detection devices, GaAs Schottky diode directly affects the noise sensitivity of the THz detecting system. In this paper, we discussed thermal noise, shot noise and the total noise voltage of GaAs Schottky diodes under different DC bias, and gave the small signal analytical solutions. At the same time, a THz mixer model was created to verify the analytical solutions. The paper is significant to study the noise mechanism of terahertz pixels and array in chip and improve the noise performance of the chip, thereby improving the sensitivity of the terahertz imaging focal plane system.