In this paper, deep etched GaAs-based micro-nano grating structures are prepared using a combination of electron-beam lithography and ICP etching technology. Electron-beam exposure deforms and distorts the mask pattern for GaAs-based gratings with small periods, long lines, and deep etching depth due to the severe proximity effect. Using thin PMMA A4 resist and SiO₂ film as multilayer resist, the spread range of electron scattering is effectively decreased and the proximity effect of an electron beam is reduced. A good photoresist mask pattern is obtained in this scheme, and the SiO₂ film is used as a hard mask to achieve deep grating etching. Furthermore, the grass phenomenon during the ICP etching process is eliminated by optimizing the RF power and adjusting the physical etching mechanism. Under optimized process conditions, the grating structure with a period of 1.00 μm, a duty cycle of 0.45, and an etching depth of 1.02 μm is obtained, and the sidewall of the grating is steep and has good periodicity and uniformity. Simultaneously, the ICP etching process’s selection ratio to the SiO₂ mask can reach 26.9∶1, allowing for the realization of a GaAs-based micro-nano grating structure with a high aspect ratio. This process provides a reference for the use of electron-beam lithography to prepare deeply etched, high-aspect-ratio GaAs-based micro-nano structures. Finally, the structure is applied to a DBR tapered semiconductor laser, and a laser output with a line width of 40 pm was obtained, indicating that the grating structure created by this process has good model selection performance for the semiconductor laser.