Complex permittivity is a crucial parameter for high-frequency substrate. Precise measurement of dielectric constant and loss is essential for the practical application of high-frequency substrate. A transmission line circuit based on microstrip lines was designed to obtain the loss characteristics of the substrate. Microstrip transmission lines of 25. 4 and 127 mm in length were simulated, fabricated, and measured to obtain the return loss S11 and insertion loss S21 within DC-20GHz. The measured data indicates that the results of S11 are below -15 dB, and the insertion loss of the transmission line is 24. 02 dB/ m at 20 GHz. By processing error analysis, the change of simulated S11max within DC-20GHz can reach about 6 dB when circuit parameters changed by 50 μm. Finally, the relative dielectric constant and loss tangent of the substrate are obtained by combining the resonant ring method with the same substrate. The results demonstrate that this method yields a high degree of accuracy for the loss tangent, with an error of less than 10% at 2, 10 and 20 GHz.