The developments of submillimeter wave and THz technologies demand test instruments at higher frequencies. The spectra of the submillimeter wave and THz signals can be measured using external mixing techniques. According to the theory of harmonic mixing, a method to measure the spectra of the higher frequency signal using external mixer at lower frequency is put forward. A W band mixer is selected to measure a signal whose frequency is about 0.2 THz. A Visual Basic program is used to analyse the test results. The results show that the method is effective to measure the spectra of higher frequency signal using external mixer at lower frequency.