The optoelectronic performance of quantum cascade detectors (QCDs) is highly sensitive to the design of the energy level structure, leading to the inability of a single structure to achieve broad wavelength tuning. To address this issue, we propose and demonstrate a modular concept for very long wave infrared (VLWIR) QCDs based on a miniband diagonal transition scheme. The modular design makes the wavelength tuning only need to be adjusted for the absorption quantum well module rather than for the whole active region. Theoretical simulation shows that the wavelength tuning range is 39.6 meV (~ 14-30 μm). To prove the feasibility of the scheme, three samples with different absorption well widths were fabricated and characterized. At 10 K, the response wavelengths of the three QCDs are 14, 16, and 18 μm, respectively, corresponding to responsivities and detectivities exceeding 2 mA/W and 1×10¹⁰Jones.