The long-term deformation performance and microscopic morphology of concrete mixed with MgO expansive agent were monitored under simulated variable temperature conditions of actual engineer， and their deformation performance under high temperature of 80 ℃ was characterized. The results show that the compensatory shrinkage performance of MgO expansive agent in concrete increase with the increase of MgO dosing under the early variable temperature course and later ambient temperature curing conditions. The compensatory shrinkage performance of concrete with high activity value MgO is more significant. When curing at ambient temperature of 3.0 years， compared with the base concrete， the deformation of concrete mixed with MgO expansive agent is basically stable. Under the high temperature condition of 80 ℃， concrete mixed with MgO expansive agent above the age of 3.5 years does not show expansion phenomenon， or even slightly contracted. With the increase of curing age， the partition interface between MgO particles and the surrounding cement paste in the concrete show a gradual integration， and the O and Mg atomic ratios in MgO particles show a gradually increasing trend.