Abstract: The acoustic properties of seabed sediments are fundamental boundary parameters for the construction of deep-sea geoacoustic models and underwater target detection. Addressing the limitations of existing in-situ measurement technologies regarding operating depth and frequency band coverage, this paper presents the design of a 6000 m-class ultra-wideband in-situ measurement system for seabed sound velocity and sound attenuation. The system integrates low-, mid-, and high-frequency source transducers covering the 1 kHz–120 kHz band and a 6-element wideband hydrophone array with non-equal spacing to form a combined vertical and oblique measurement approach. To overcome the challenge of difficult signal first-arrival picking in complex detection environments, a hybrid algorithm combining STA/LTA and AIC is introduced to achieve precise extraction of first-arrival time. Anechoic tank experimental calibration results demonstrate that the system exhibits high measurement stability across the entire 1 kHz–120 kHz frequency band, with the sound velocity measurement precision (RSD) controlled within 0.5%, the error range is under ±10m/s and the accuracy better than 0.15% (<2.3m/s, comparison with results from high-precision sound velocimeter). The system adopts a comparative attenuation method based on tank reference subtraction to compensate for the influence of transmit-receive directivity on attenuation measurement. The fluctuation of the calibrated geometric inherent attenuation coefficient decays significantly as the measurement distance increases.