For this investigation, selected were some coin samples of several important phases in the coin’s developing procedure, and the microstructure and composition of coin alloy were studied by using optical microscope, scanning electric microscope, and X-rays energy spectrum instrument. The data reveal the characters of coin alloys during various historical periods and indicate that there are identical results with chemical method. The tin content of Qi knife-coins was high in the early periods, but at the same time the lead content increases and was higher than tin, and the alloy component became more stable. It indicates that coin alloy composition had been unified at that time. With the historical evolution, the brass coin alloy contained tin in the early periods changed gradually to the ones without tin. Lead content tended to decrease, and zinc content increased gradually, the brass coin alloy microstructure changed from [alpha] phase brass to [alpha]+[beta] phase brass, and there after finished the changing procedure from lead-tin bronze to brass alloy and matured brass alloy.