Wavelength is a crucial parameter for terahertz sources, and the Fabry-Perot (F-P) interferometric method is the preferred approach for measuring terahertz-source wavelengths. However, employing a high-resistance silicon wafer as the interference-cavity plane mirror presents challenges such as low reflectivity, a narrow applicable frequency band, and imprecise stripe precision; consequently, the demands of high-precision wavelength measurements cannot be satisfied. Hence, a series of interference chamber silicon-grid plane mirrors, which comprises high-resistance silicon wafers plated with a periodic metal grid, were designed and processed. These mirrors demonstrate a reflectivity exceeding 87% in the 0.08?0.53 THz band. The F-P interferometer successfully measures the wavelengths of 0.096 THz/0.14 THz avalanche sources and a 0.315 THz Schottky source, where optimal measurement results are yielded with wavelength errors of 0.16%, 0.33%, and 0.54%, respectively. Compared with measurements using high-resistance silicon wafers, this approach significantly improves the interference stripe precision and the half-peak width of the transmission peak. The simplicity of the structure facilitates ease of fabrication, thus contributing significantly to advancements in terahertz-wavelength measurement accuracy and related terahertz devices.