Moisture content is critical to concrete fabrication. Currently, concrete fabrication often involves experienced workers who can gauge whether the water volume is sufficient. This active human involvement requires appearance or thickness checks, which is not suitable for mass production. This method cannot also tell the actual moisture content. Moreover, different workers may give different assessments for the same concrete, which may further lead to different strengths of concrete. Thus, we propose and design a microwave-cavity-based online moisture sensing system to monitor concrete fabrication. Our system utilizes a microwave resonator as a front-end sensor to monitor moisture. This resonator is sensitive to the dielectric coefficient of a particular object. This dielectric coefficient of concrete notably changes when the water yield varies and hence can be readily detected. With this sensitive resonator, we built an online moisture-sensing system. Our prototype achieves moisture sensing with an error of as low as 0.5% and can check moisture content twice a second.