Photocurrent spectrum for GaAs/AlGaAs multiquantum well structure was measured at 77K. A strong photocurrent peak at ν =1589cm -1 ,i.e. λ =6.29μm,in the photocurrent spectrum was observed. It was believed that the strong photocurrent peak is relevant to the interference of electrons above the barrier of multiquantum well structure. The calculated position of peak of photocurrent on the basis of theory of electron interference is in very good agreement with the experimental results.