The thick sandstone with an upper layer of clay (2 m to 3 m) was identified at the transmission line projects in western China. Tower foundations are affected by extreme weather such as strong wind and snow all the year round, so the research of foundation uplift is very significant. Belled piles are widely used in the area due to its high uplift bearing capacity, however, further research studies regarding the uplift bearing capacity of the belled piles in this special scenario still need to be conducted. The uplift bearing characteristics of five rock-socketed belled piles were studied by field testing method. The load-displacement curves, fitting analysis of load-displacement curves, change rate of pullout stiffness, distribution of axial force alone the pile, and pile side friction were analyzed. Observation results show that the load-displacement curves of rock-socketed belled piles are steeply changed. Moreover, we find that the hyperbolic model can provide a good fitting result for load-displacement curves, and the ultimate uplift bearing capacity is suggested to be predicted by using a reduction parameter of 0.64–0.85. Both uplift bearing capacity coefficient N_u and utilization ratio of pile material η increase with the increase of L/d under a same diameter, while the growth rate decreases. In this paper, the significant size effect is observed with increasing of pile diameter. The change rate of pullout stiffness-displacement curves are consistent with the trend of the safety factor-displacement curves, and can be fitted by power function model. Additionally, the change rate of pullout stiffness at the limit state will decrease to around 154 in this study, which can be used as the instability threshold for the rock-socketed belled piles in a similar soil foundation.