Rapid growth behavior of ζ phase has been investigated in the undercooling experiments of Cu-14%Ge, Cu-15%Ge, Cu-18.5%Ge and Cu-22%Ge alloys. Alloys of the four compositions obtain the maximum undercoolings of 202 K(0.17T L), 245 K(0.20T L), 223 K(0.20T L) and 176 K(0.17T L), respectively. As the content of Ge increases, the microstructural transition of “α(Cu) dendrite + ζ peritectic phase → ζ peritectic phase → ζ dendrite + ( +ζ) eutectic” takes place in the alloy at small undercooling, while the microstructural transition of “fragmented α(Cu) dendrite + ζ peritectic phase → ζ peritectic phase → ζ dendrite + phase” happens in the alloy at large undercooling. EDS analysis of the Ge content in ζ peritectic phase indicates that undercooling enlarges the solid solubility of α dendrite, which leads to a decrease in the Ge content in ζ phase as undercooling increases. In the Cu-18.5%Ge alloy composed of ζ peritectic phase, the Ge content in ζ phase increases when undercooling increases, which is due to the restraint of the Ge enrichment on the grain boundaries by high undercooling effect.