The rapid solidification and microstructure formation of Cu-Pb monotectic alloy were investigated by melt spinning method. The relationship between liquid separation and physical parameters such as cooling rate and solidification time etc. was analyzed theoretically by coupling the heat transfer equation and Navier-Stokes equation. It is revealed that the rapid cooling of the melt has more remarkable effect on microstructure forming of the monotectic alloys than liquid flow under rapid solidification. The rapid solidification can suppress the liquid separation, which results in the formation of uniform microstructure for various Cu-Pb monotectic alloys. With the rise of cooling rate, the grain size clearly decreases, the microstructure was markedly refined and the microstructural morphology changes from coarse dendrite to uniform and fine equiaxial crystallites. Increasing the cooling rate and shortening solidification time are the essential conditions to suppress liquid phase separation and obtain the uniform monotectic microstructure in gravity field.