Abstract: An important cause for the quality of cementing high temperature and deep wells to be difficult to maintain is that the integrated performance of a cement slurry at elevated temperatures is difficult to maintain. Presently the high temperature performance is generally adjusted using cement additives, which are generally polymers. The settling stability and filtration rate of a cement slury at high temperatures are maintained with filter loss reducers, while the thickening time of a cement slurry is adjusted with retarding agents. As a well is drilling deeper and deeper, the downhole temperature and pressure are increasing, and it becomes more and more difficult to control the settling stability, filtration rate and thickening time of a cement slurry. Even so the methods of evaluating the performance of these polymer additives are rarely seen. A method is recommended for use in evaluating the thermal stability of polymer additives. This method is based on the mechanism of viscosity reduction of polymers at elevated temperatures and an Ubbelohde viscometer is used to measure the change of viscosity. By measuring the rate of change of molecular weight of additives before and after aging at a certain high temperature, △M, the thermal stability of a polymer can be measured; when △M is approaching zero, the thermal stability is good. This measurement can be used to guide the selection of high temperature cement slurry additives.