Thermal variation of the Co79.5Sn20.5 alloy during the cooling process was analyzed by the differential scanning calorimeter (DSC) and its equivalent magnetic susceptibility change during the process was measured using a new device. The magnetic susceptibility and the heat of the alloy will change abruptly when there are structure changes. During the disorder-order phase transition in the solid state, the structure and the equivalent magnetic susceptibility were studied. Moreover, the comparison among the microstructure changes by different physical properties indicates that the solid-liquid transition, the Curie transition and the disorder-order solid transformation can be well characterized by the variations of the equivalent magnetic susceptibility.