Abstract: To address the distortion of cutting force, vibration, and temperature along complex transmission paths in milling, and the difficulty of high-fidelity synchronous acquisition, a multi-field synchronous sensing tool holder system for in-situ cutting monitoring was designed and implemented. Based on a standard BT40 tool holder, the internal cavity was designed according to the layout of sensing units and the available installation space. Parametric modeling, finite element analysis, and multi-objective optimization were then conducted to balance structural strength, stiffness, and dynamic balance. Force, vibration, and temperature sensing units, together with data acquisition circuits, were integrated inside the tool holder to form an in-situ multi-source synchronous sensing scheme for the cutting process. A 2.4 GHz Wi-Fi link was used for wireless data transmission during cutting. Experimental results show that the measured cutting force agrees well with that of the reference system, with an NRMSE of 8.57% and an overall deviation of less than 10%. In the frequency range of 200–1000 Hz, the average noise level of the internal vibration measurement is approximately 2.7 dB lower than that of the external sensor. Temperature calibration results show deviations within ±1 ℃ in the low-temperature range and within ±2 ℃ in the medium- and high-temperature ranges. These results verify the in-situ sensing and stable transmission capability of the system.