A novel grating-based spectrometer system without any light filter was proposed. In order to obtain a pure spectrum, instead of inserting several filters into the light path in turn mechanically in the conventional grating-based spectrometer, the key features of the new method were measuring the relative diffraction efficiency between the high-order spectrum and the first-order spectrum of the grating-based spectrometer system, and designing the algorithm, which could subtract the unwanted high-order and spurious spectrum. The verification experiments were carried out in the wavelength range of 240–810 nm with satisfactory results. This method is particularly useful for a high-speed full spectrum scanning system with a multi-channel array detector and meets the requirement of high-speed parallel spectral measurement.