The density detection of liquid products is one of the important means to audit product quality. Aiming at the problems of large volume, high power consumption and difficulty of online detection of the current traditional density detection device, a liquid density measurement sensor based on piezoelectric micromachined ultrasonic transducer (PMUT) was designed and fabricated in this paper. The sensor consists of two PMUTs (the radius is 500 μm, and the resonant frequency in air is 136 kHz), which are used as the transmitting end and the receiving end, respectively. The experimental test results show that the resonant frequency of the sensor has a linear relationship with the liquid density, it has a good response in the density range of 776 to 1 150 kg/m3, and the sensitivity is 17 Hz/kg/m3.