In the field of room-temperature terahertz (THz) wave detection, the laser-pump frequency up-conversion method is far better than the traditional thermal detector due to its high sensitivity and fast responsivity, which greatly improves the signal-to-noise ratio of the active THz system. According to the three-wave process theory in nonlinear optics, the responsivity of the up-conversion process partly depends on the phase-matching conditions in the nonlinear optical crystal. This paper studies the influence of the wavelength of probe light in the up-conversion detection based on homemade organic crystal DAST, and provides the optimum probe wavelengths. By applying optimized probe wavelengths, compared with the fixed probe wavelength (e.g., 1356.9 nm), the detection responsivity is increased by 291%, 298% and 122% at 16.51, 19.23 and 26.06 THz, respectively. We believe the presented technology can promote the applications of THz source in the fields of high-resolution spectroscopy and imaging.