DoubleFrequency Automated Test Technique can detect and identify all receive channels and interference paths of receivers rapidly, obtain nonlinear response characteristics of receivers, and estimate electromagnetic environment adaptability of receivers thoroughly. But the test result would be influenced by harmonics of signal sources, which makes estimation to the receivers different from the truth. To ensure reliability of the test result, it is necessary to set reasonable requirement for signal sources’ harmonic suppression. By mathematical deduction and simulation of receivers’ responses, this paper analyzes the influence of signal sources’ harmonics, proposes and verifies how to choose harmonic suppression of signal source when constructing DoubleFrequency Automated Test System. That is, for general receivers, harmonic suppression of signal source should be better than -60 dBc for the worst case to identify receivers’ receive channels and interference paths while using DoubleFrequency Automated Test.