The navigation monolith of Longtan RCC dam is 88 m long along the dam axis. In this monolith, 2 transverse joints were set up in the lower part with no longitudinal joint . The second-stage cooling for joint grouting was not required. The temperature would drop extremely slowly inside the dam body and the axial stress on upstream-surface would be very high, so that the vertical cracks on upstream surface could hardly be avoided. The authors conducfed a 3- D simulation of the 3-D temperature field and stress field under several load-cases for this monolith during the construction process. The calculation results showed that , even if a series of comprehensive temperature control measures are adopted without any insulation on the upstream surface, 2.0 ~ 2.2MPa tensile stress in dam-axial direction on the upstream-surface will occur in winter under the effect of yearly change of air temperature. 3.0MPa or higher stress may be caused by cold wave. The total stress would be 5.0 MPa or higher, which is high enough to produce exterior cracks. Deep vertical cracks on the upstream surface can be induced after reservoir impounding. The results also showed that the tensile stress on upstream surface can be reduced to less than 1.6MPa by sticking a layer of 4~ 5cm thick polystyrene plate on the surface, which can effectively avoid the exterior cracks as well as deep vertical crack on the upstream surface.