The present paper put forward a new method, named one step method, to design concave holographic grating for a monochromator. This new method is simple and direct and easy to understand. Additionally, in this new method, we can control the whole aberrations of concave grating very well. Genetic algorithm was applied to optimize the objective function of this new method for its strong ability to search the extremum of nonlinear functions and a comparison was made between this new method and the classical method. The result shows that, for coma correction or astigmatism correction, the imaging properties of the concave grating designed by the new method is much better than the grating designed by the classical method.