In this paper, a high gain and high fully differential gain boosted operational fold cascade amplifier is proposed. The amplifier is designed for MDAC of pipelined analog-to-digital converter. Both the main amplifier and the boosted amplifier adopt fully differential fold-cascade structure. The main amplifier uses a switched capacitance common mode feedback circuit to stabilize the output. With 5.0V power supply, this circuit is designed in CSMC 0.5μm CMOS process. Spectre simulation shows that the whole amplifier has the DC gain of 126.3dB and the unity gain bandwidth of 316MHz under 2pF single ended load, and the settling time is 4.3ns with an accuracy of 0.01%.