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An effective method for calculating the Discrete Fourier Transform (DFT) of a double-sampled sequence (a pair of the equi-interval-sampled sequences) is presented in a matrix form. The calculation formula shows that the DFT of a certain type of band-limited signals can be effectively evaluated from the DFT's of a pair of the equi-interval-sampled sequences of the signal. It is demonstrated that the double-sampling technique in digital signal processing produces the effects similar to the beat-down in a heterodyne system of analog circuits. A calculation example is given in detail and some computational and practical applications are described briefly.