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The highlight of in vivo measurement to monitor abdominal compartment syndrome (ACS) caused by surgery has emerged since ACS without care will lead to death and organ failure if ignored in advance. Typically, it is intuitive to conventionally implant a monitoring electronic device or swallow an electronics device (which is an emerging technology) such as pressure sensor in body for a long time to continuously detect the abdominal compartment abnormal pressure; however, the pressure sensor in the body may have risk. In this letter, we design and fabricate a flexible capacitance sensor to detect the tiny capacitance change due to the acidity (pH value) in the saliva caused by the ACS. The devised circular-type sensor senses the 2-fF capacitance change in air, 8-fF change in deionized water, 12-fF change in glucose liquid, and 14 fF in 0.5% hydrochloric acid. The proposed highly sensitive capacitance flexible sensor is not only used to detect saliva pH change caused by ACS but can also extend to a wearable device to monitor human biomarkers, such as pH and Na+ in sweat, by integrating the microfluid device.