Abstract: A cement slurry used to deal with difficulties cementing the exploratory well Songke-2 in main land China, has been formulated with a quadripolymer filter loss reducer and a terpolymer phosphonate retarding agent to improve its thermal stability and avoid the risks of "thermal thinning" of the cement slurry. The filter loss and thickening time of the cement slurry were controlled to satisfy the needs of cementing ultra-high temperature wells by adjusting the concentrations of the two additives. Meanwhile, based on particle sizing and the close-packing principle, the particle sizes and concentration of silica sand were optimized, adjusting the ratio of silicon over calcium closing to 1, thereby preventing the decaying of the late-stage strength of set cement at ultra-high temperatures. Furthermore, an elastic tough material developed with particles and fibers was used to enhance the elasticity and toughness of the cement slurry. By optimizing the ratio of these additives, a cement slurry able to tolerate 260℃ well temperature was designed. This cement slurry has good stability, density difference between the upper and the lower parts of the cement slurry less than 0.03 g/cm³, thickening time between 200 min and 420 min, filter loss less than 100 mL, 48 h compressive strength greater than 20 MPa, and 7 d compressive strength greater than 38 MPa. The late-stage strength of the set cement is not declining. By optimizing the liner hanging and cementing techniques, and strictly controlling cement slurry density, the integrity of the formations penetrated by the well was maintained, without being fractured during cementing operations. Using high temperature high efficiency flushing spacers, the displacing efficacy was increased, and job safety and cementing job quality were ensured. Well cementing has been performed successfully with high job quality on the Well Songke-2, whose static bottom hole temperature is 260℃, and circulating bottom hole temperature 210℃.