The invention provides a preparation method of a composite ultralow-temperature phase-changed cool-storage agent. The preparation method comprises the following steps that firstly, potassium chloride, sodium chloride, ammonium chloride and water are prepared; secondly, potassium chloride and sodium chloride are added into the water and stirred for dissolution, and a solution A is obtained; thirdly, the solution A is heated, ammonium chloride is added while the solution A is stirred, stirring is conducted to be uniform, and the composite ultralow-temperature phase-changed cool-storage agent is obtained after cooling. Through reasonable matching of the cool-storage agent components and reasonable proportions, the composite ultralow-temperature phase-changed cool-storage agent or a composite ultralow-temperature phase-changed cool-storage device has the advantages of being low in phase-changed temperature, high in phase-changed latent heat, low in cost, free of toxins, odourless, safe, reliable, environmentally friendly, convenient to store and unrestricted in air transportation.