Based on 10 years of prototype monitoring data of existing MgO concrete arch dams in China, such as the Sanjiang Arch Dam, and in combination with measured temperatures under the boundary conditions in regard to reservoir water, air, and foundation, the temperature field and temperature load of arch dams are discussed. Observed data from the Sanjiang Arch Dam using the plumb line and inverted plumb line method indicate that the deformation of the arch dam induced by adding MgO to concrete for dam construction is similar to that caused by temperature rise, verifying the expansion effect of MgO concrete. From the viewpoint of the macroscopic deformation of the arch dam, the engineering significance of MgO concrete is further discussed.