Thickening characteristics is an important behaviour of cement slurry in the operation of geothermal well cementation. The thickening tests have been carried out under the simulated conditions of temperature and pressure. The appraisal has been made of seven kinds of retarders through high temperature retarding tests, the results show that Boric acid is an effective retarder for high temperature cement slurry, which is employed in regulating the consistency of two high temperature geothermal cement slurries.