In order to broaden the wavelength range of flat-field holographic concave grating (FHCG), the optimization method of parallel flat-field holographic concave gratings (PFHCGs) is proposed. Two flat-field gratings working in different wavebands, one for 2~5 nm and the other for 5~30 nm, are designed on the same substrate based on genetic algorithm, and the two gratings can work simultaneously because of the same working structure. Theoretical spectral resolution of PFHCGs is studied via ray-tracing method, which is pretty the same as that of existing commercial FHCG. Based on the optical path function theory, the recording optical path of PFHCGs is given. The results demonstrate the line density deviation is less than ±1 line/mm between theoretical value and optimization value. Simulation results show that the waveband of HPFCGs is broaden and the resolution is also ensured, so it is suitable for recording wider waveband of soft X-ray.