The invention discloses a quantum well terahertz detector which is composed of a multiple quantum well chip and a superconducting magnet system. Interaction between a donor level in a barrier layer in the multiple quantum well chip and a sub-band energy level in a potential well layer is regulated effectively by exerting an applied magnetic field so that electrons are transferred from a ground state sub-band energy level in the potential well layer to the donor level in the barrier layer, and incidence THz radiation is detected by utilizing electron transition between the donor levels in the barrier layer. According to the quantum well terahertz detector, after the applied magnetic field increases to a critical magnetic field Bc, due to the fact that THz detecting can be conducted by utilizing electron transition between the donor levels, the quantum well terahertz detector is free from grating coupling or angle lapping coupling of 45 degrees, the THz radiation is absorbed and responded under the condition of normal incidence, the defects caused by principle of an existing quantum well structure detector are overcome, and responsivity can be improved substantially.