Currently, high-frequency terahertz (generally referring to 30–100 THz, also known as mid-infrared) detection in the mid-infrared (MIR) range at room temperature faces challenges such as limited detection sensitivity and a lack of ideal semiconductor materials. A nonlinear optical upconversion method offers a new approach to high-frequency THz detection. We conducted an experimental study using a ~ 1 μm laser to pump BaGa4Se7 crystal for 37.5–100 THz high-frequency THz upconversion detection. We constructed an active coherent high-frequency THz system, including an optical parametric oscillator (OPO), upconversion components, and near-infrared photodetectors. The study mainly analyzed the effect of gain length on difference-frequency generation (DFG) detection and compared the upconversion light output intensity under different gain lengths with respect to the input of probe and MIR. By increasing the number of detection crystals to extend the length of the nonlinear interaction. The upconversion light intensity with two detection crystals was enhanced by a factor of 1.5–2.7 compared to using a single crystal. This verifies that a longer effective gain length leads to higher upconversion energy, improving the responsivity of DFG. Frequency upconversion detection is widely applied in spectroscopy, imaging, and biomedical detection. Increasing the upconversion light intensity for better detection is a goal across various fields. Our work provides references for system design, with potential for expansion to other high-frequency THz upconversion systems.