Hydrogen isotope behavior in tungsten coated on reduced activation ferritic/martensitic (RAFM) steels such as China low activation martensitic (CLAM) steel has attracted more attention in the fusion engineering research community. This paper is mainly devoted to the investigation of the effect of tungsten coating on deuterium permeation and retention behavior in RAFM steels. The permeability and diffusion coefficients of CLAM, W-CLAM, and W were determined by gas-driven permeation (GDP) tests followed by thermal desorption spectroscopy to measure deuterium retention. It was found that the observed deuterium permeability and diffusivity of the composite W-CLAM specimen was reduced to about ~60% of the pure CLAM steel, whereas deuterium retention increased, evidently owing to the W coating on the surface that caused the slower release of D into the environment and increased of the effective surface area. In addition, a key finding was that the lath martensite–coarsened and more precipitate phase was found, which may be due to the migration of lath interface during the GDP test.