The terahertz (THz) wave contains abundant spectrum resources and is still in the early stages of development. It has great application potential in biomedical engineering and public security. However, in these areas there are difficulties to overcome like measuring the wide band absorption of a trace mount sample. In this paper, a THz absorption enhancing method is suggested by a multiplexing strategy. By gradually expanding the stretchable substrate of the dielectric metagrating with an oblique THz wave incidence, the resonance peak frequencies can cover the frequency range of 0.48-0.58 THz. Also, the corresponding envelope built by the peaks of the metagrating absorption spectrum with the 0.2 µm α-lactose film can demonstrate 71.55 times boosting compared to the original absorption amplitude of the film. The investigation witnesses possibilities for the detection of biomacromolecular materials.