Second-harmonic interferometry (SHI) is proposed for measuring the electro-optic (EO) coefficients of massive media. It combines the advantages of interferometric techniques with the mechanical stability of single-beam methods, simultaneously skimming the wavelength dispersion of the EO response. For demonstrating the effectiveness of the SHI technique, the EO coefficients $r_{33}^T$ and $r_{13}^T$ of the EO crystal lithium niobate are measured simultaneously at 1064 and $532\mathrm{nm}$ .