A tungsten (W)-doped high density carbon (HDC) target is a promising design for an ignition target in inertial confinement fusion (ICF). The influence of silicon (Si) and W on the transmission of M-band x-ray has been studied by experiment. With a radiation temperature of ∼200 eV, the transmitted M-band x-ray (1.6–4.4 keV) flux and spectrum of Si or W-buried HDC sample were measured by M-XRDs and TGS, respectively. The thin layer of Si or W was buried at two different depths (2.1 μm and 11 μm). Results show that M-band transmission flux of Si-buried HDC sample decreases with buried depth (bd). However, bd does not influence that of the W-buried HDC sample. The one-dimensional simulation result is consistent with the experimental result. In the 1D simulation, the Au M-band (2–5 keV) transmission flux of Si-buried HDC also decreases with bd. However, the Au M-band transmission flux of W-buried HDC increases at first and then decreases. This is mainly due to the different characteristics of Si and W opacity. Especially as the peak radiation temperature reaches 260 eV, W can still absorb the M-band x-ray efficiently as it is buried near the radiation source. Based on these studies, an optimized W-doped HDC target with low doped fraction and mass has been proposed in this paper.