Abstract: Electrolytic Though Pitch Copper （Cu-ETP） is often used to fabricate microwave resonator, which is the core component of primary thermometers. Its isothermal compressibility is very important for high accuracy thermodynamic temperature measurement. Due to the lack of physical property data of Cu-ETP at cryogenic temperature, the isothermal compressibility of Oxygen-Free High-Conductivity copper （OFHC） is often used to approximate the isothermal compressibility of Cu-ETP. In order to obtain the in-situ experimental measurement results of the isothermal compressibility of Cu-ETP, the isothermal compressibility of Cu-ETP at the triple point of neon （24.556 1 K） is measured using the multi-mode microwave resonance method based on the measurement system of single-pressure refractive-index gas thermometry, and the uncertainty is calculated. The final result is in good agreement with the OFHC isothermal compressibility obtained by indirect calculation method, and the uncertainty level is similar. A set of extrapolated values of Cu-ETP isothermal compressibility in the temperature range of 5 K to 24 K is obtained by referring to the isothermal compressibility of Cu-ETP measured in-situ at the triple point of neon. The results can provide reference data for thermodynamic temperature measurement and basic thermal properties measurement of gas in cryogenic temperature region.