Abstract: With the advancement of petroleum exploration and development technologies, especially with respect to materials and control, the experimental study of rock physics is moving towards in-situ deep surface layers under ultrahigh temperature and high pressure. This paper presents a seismic elastic parameter measurement system for ultrahigh-temperature and high-pressure rocks. This system utilizes traditional presses and graphite tubes to achieve temperature and pressure control, and has the ability to quickly load ultrahigh temperature and high pressure, so it can be used for experimental study of rock physics under ultrahigh temperature and high pressure conditions. With this system, 10 rock samples were tested to obtain the elastic parameters such as acoustic velocity under ultradeep reservoir conditions (temperature ~250 ℃, pressure ~300 MPa). Moreover, the experimental data were fitted over a wide range of pressure and temperature domains, the temperature-pressure dependence of elastic wave velocity was analyzed, and the covariant relationship between velocity and temperature-pressure was characterized. The results indicate that the velocity decreases and increases logarithmically with the increase of temperature and pressure respectively, and the longitudinal wave fitting coefficient has a clear directional effect on the pore type.