The contradiction between spectral resolution and throughput for the optical spectrometers is still a problem that needs to be solved. We introduce a simple and feasible method of the digital projection slit (DPS), which can improve both the spectral resolution and throughput of the optical spectrometer. The DPS spectrum is accurate and reliable without using the optical transfer function (OTF) of the optical spectrometer. The method has been successfully applied in the fiber spectrometer and the Raman spectrometer. The resolution of the recovered spectra can be increased by ~3 times and the throughput can be increased by ~5 times.