We present an approach to achieving high-resolution spectrometry through the utilization of a double transmission grating, in which two planar transmission gratings are strategically placed at an angle. This arrangement enables the light to pass through the dispersive element twice, resulting in a double dispersion and significantly enhancing the spectrometer’s resolution. We rigorously derived theoretical formulas for dispersion and parameters of the double grating model. Simulation results indicate that employing two transmission gratings with a groove density of 900 l/mm, the transmission-double-grating spectrometer can effectively measure wavelengths from 825 to 900 nm, the simulated resolution results in for 0.11 to 0.12 nm and the experimentally tested full width half maximum is 0.12 nm. In addition, a prototype of the spectrometer has been built and tested, and the experimental results are in general agreement with the theoretical modeling and simulation.