As a common rock layer in underground engineering, thermal deformation under the action of thermal mechanical coupling of limestone is crucial in high-temperature underground engineering such as deep nuclear waste storage, underground gasification of coal, and geothermal resource development. In this experiment, the evolution of thermal expansion of limestone under high temperature triaxial stress state was investigated by using servo-controlled multifunctional triaxial test system. The experimental results show that the triaxial stress compresses the fracture closure and inhibits the expansion of mineral particles during the real-time warming process under high-temperature triaxial stress, and the microfractures are accompanied by continuous opening and closing competition relationship with the increase of temperature, and the limestone thermal strain curve is step-like, and the thermal strain size and step fluctuation gradually decrease with the increase of hydrostatic pressure. Under the action of thermal mechanical coupling, the inhibition of thermal expansion coefficient is enhanced by triaxial stress at high temperature stage due to thermal cracking and softening of limestone solid skeleton under the action of temperature. There is a threshold pressure for the average coefficient of thermal expansion of limestone during the warming process at 500°C. When the hydrostatic pressure is not less than 25 MPa, the coefficient of thermal expansion decays significantly.