To realize flat-field concave holographic grating with high resolution and broad spectral region, Namioka concave grating theory, genetic algorithm, simultaneous free-aberration and common diffractive orders are combined to produce the ideas and method of designing flat-field concave holographic grating. A configuration simulation based on flat-field concave holographic grating is presented, whose root mean square (RMS) spot radius of spot diagrams is given. The RMS spot radius is around 11 μm in full spectral region 200~800 nm. While a 10 μm×1 mm entrance slit is used, the illumination spectrum image show that the resolution is 0.25 nm and 0.5 nm in 200~400 nm and 400~800 nm spectral region, respectively. All prove that these ideas and methods are available in designing compact and utilitarian spectrometer.