We study theoretically the plasmon excitations in a two-dimensional electron gas (2DEG) with spin-orbit interactions (SOIs) embedded in a (1 1n) crystallographic plane.We demonstrate that the energy spectra and dielectric functions between the 2DEGs embedded in different crystallographic planes can be related by a unitary transformation.Using the unitary transformation,we find that the anisotropy of plasmon excitations and the directional plasmon filtering (DPF) can be tuned by changing the strengths of SOIs in the high-index planes.There are two advantageous directions [1(1)0] and [nn(2)] for plasmon propagation.Moreover,the anisotropy and the DPF can be smeared out by tuning the strength ratio α/β between the Rashba SOI and the Dresselhaus SOI.