In order to study the effect of magnesium oxide expansion agent and hydration heat regulator compounding on the crack resistance of concrete and explore the mechanism of hydration heat regulator, diabetic temperature rise, limited expansion rate, volume deformation, self-shrinkage, elastic modulus of the concrete mixed with magnesium oxide and the concrete mixed with magnesium oxide and hydration heat regulator were tested, and the microstructure was investigated. The results show that the hydration heat regulator can significantly change the cement hydration exothermic process, the temperature suppression rate of the slurry reaches 31.2%, and the temperature suppression effect is enhanced with the increase of the mold temperature. The hydration heat regulator can stimulate the hydration of magnesium oxide and promote the release of expansion energy, and the effect is more obvious with the increase of curing temperature. The hydration heat regulator can change the temperature rise history of concrete, especially reduce the early hydration heat release rate. The combination of hydration heat regulator and magnesium oxide can reduce the self-shrinkage of concrete. When the content of hydration heat regulator is 0.2% and 0.4% (mass fraction), the shrinkage value is reduced by 84 με and 54 με, respectively. The hydration heat regulator will reduce the elastic modulus of concrete in the early age, but it has little effect on the elastic modulus in the long age. The change of cement hydration process, hydration product generation and microstructure compactness caused by hydration heat regulator is the fundamental reason for improving the crack resistance of concrete.