An ultra-sensitive MEMS piezoresistive pressure sensor and its optimization theory were presented for ultra-low pressure measurement.The pressure sensor was designed with working range of 0-500 Pa and had a high sensitivity of 25 mV/V.The large sensitivity was obtained due to novel diaphragm combined peninsula-island structures.These structures made stress gradient along the boundary of SCR (stress concentration region) change sharply which enabled little energy to be wasted outside SCR.And also the trade-off between high-sensitivity and linearity was avoided.In comparison to a flat diaphragm with the same size,the proposed sensor structures could achieve a sensitivity increased by 259%,nonlinearity reduced by 79%,and resonance frequency kept approximately.