Given the importance of the 0.1- to 1-THz range in the security detection and diagnosis of cancer, we propose a structure for graphene-based terahertz (THz) photodetectors. We use the finite-difference time-domain and mathematical methods to calculate the light absorption in the graphene layers (GLs) and the detector’s figure of merits, including responsivity and detectivity. The proposed photodetector is based on single-layer and multilayer graphene absorbers within a p-type-intrinsic-n-type (PIN) photodiode structure. This detector is designed to achieve maximum absorption at the THz frequencies by creating adjustable Fabry–Perot resonances. Results show that it has two distinct and adjustable detection peaks in THz. So we set the operating frequency to 0.35 and 0.83 THz by adjusting the geometric parameters of the device, which also fit the atmospheric windows. Despite the low number of GLs, high absorption is provided at room temperature. It has high responsivity peaks of ∼1220  AW^−  1 at the frequency of 0.35 THz and 420  AW^−  1 at 0.83 THz, respectively, while we have achieved these characteristics with fewer GLs than other previous detectors.