The influence of expansive volume of concrete mixed MgO in different parts on integrated structure and the control to its delaying time of expansion are the two key problems in MgO concrete technique. The effect of the thermal stress of concrete mixed MgO pouring block was studied. Mixing with MgO in concrete can make tensile stress lessen in structure and the stress appears earlier than ordinary concrete slightly. The more the MgO content, the less the tensile stress. It is important to ensure that the swelling of MgO must be coincident with up - down different lifts of puring blocks. The results show the properties of thermal stress of concrete mixed MgO.