Driven by the fast development in optical fiber interconnections, there exists an increasingly growing demand for wider bandwidth. DWDM technology can extremely enhance the number of wavelengths (de)multiplexed. Among many DWDM devices, free-space diffraction gratings (FSDG) play an important role, with the advantages of parallel and athermal processing as well as low polarization-dependent losses. In our work, we describe by means of the rigorous coupled-wave analysis (RCWA) that high-density holographic photoresist gratings, if being in optimized profile parameters, can realize high diffraction efficiencies, e.g., the first Bragg diffraction efficiency can theoretically achieve more than 90% both in TE and TM polarizations. And we have successfully fabricated the 600 lines/mm photoresist gratings whose first Bragg diffraction efficiency can experimentally reach 80% with the grating depth 2.9μm. The experiment process is presented in detail, and the experimental result is in good agreement with the theoretical one.