The solidification processes of twin-roll strip casting Cu-3.2 Ni-0.75 Si alloy at different cooling rates were observed in situ by confocal laser scanning microscope（CLSM）,and the functional relationship between the crystallization temperature and the secondary dendrite spacing of the solidified structure depending on cooling rate was quantitatively analyzed.The results show that the increase of cooling rate can significantly reduce the crystallization temperature and the secondary dendrite spacing of the solidified structure. When the cooling rate increases from 0.1 ℃/s to 100 ℃/s, the crystallization temperature decreases from 1 096 ℃ to 890 ℃,and the secondary dendrite spacing increases from 77.5 μm to23.5 μm, the function relations of crystallization temperature and secondary dendrite spacing depending on cooling rate are TL→ S=194×（-v/14）+895 and λ2=60×v-0.11, respectively. The subrapid solidification characteristics of twin roll strip casting can significantly refine the solidification structure of the alloy and inhibit the microsegregation of Ni and Si elements. The reason is that the increase of cooling rate enhances the initial undercooling of the alloy melt during crystallization.