With the development of energy exploration and deep exploration in China， there are more and more deep wells， and the formation temperature and pressure are higher and higher. To ensure the stability of rheological properties of drilling fluid under high temperature and high pressure is a prerequisite for safe drilling in deep wells. The determination of rheological properties of drilling fluid under high temperature and high pressure is the most basic work for evaluation and optimization of the drilling fluid system. With the support of the national major equipment and instrument development project of the Ministry of Science and Technology of China， the first ultra high temperature and high pressure rheometer in China has been successfully developed by tackling the technical problems with the pressure control system， temperature control system， shear rate control system， viscosity detection system， etc. The instrument can measure the rheological properties of drilling fluid， completion fluid and fracturing fluid with high precision under high temperature and high pressure， as well as low temperature and high pressure.