The effect of Bi-content on microstructure evolution of Al-Bi monotectic alloy and solidification progress ofmiscibility gap under the action of gravity was studied by air cooling to desired temperatures and thenquenching, as well as bynumerical simulation. The average diameter and volume fraction of Bi-rich droplets in Al-10%Bi monotectic alloy are 1.1 and 12.8 times of those in Al-5%Bi monotectic alloyrespectively. A huge amount of large-size drops rich inBi gather in the lower part of Al-10% Bialloywith low solidification uniformity.The average size of drops rich in Bi in the alloy of Al-5%Bi with a uniform solidification; There's no large-size drops rich in Bi in the solidifiedmonotecticalloy with Al-3.4%Bi, and its solidification is uniform.The analysis demonstrates that the increase of the amount of Bi-rich phase may enhance the gravity action on the drops, thereby aggravate the solidification process with separation and decomposition of liquid phasesin the immiscible area, and enlargethe size and number of drops rich in Bi,whilethe synergistic action of Ostwald ripening and collision coagulation of drops yielda huge amount of large-size drops rich in Bi gathering in the lower part of the solidifying alloy, thus aggravates the macro segregation of the solidified monotectic alloy.