<p>This paper demonstrated the feasibility of utilizing miniaturized piezoelectric micromachined ultrasonic transducer (PMUT) to form a symmetric V-shaped acoustic beam pattern, which enables the ability to synchronously transmit bidirectional ultrasonic signals, and offers a promising technology to address the frame rate limitations in traditional ultrasonic flowmeters based on time-of-flight (ToF). In contrast to the previous two-step flow rate monitoring scheme, where paired ultrasonic transducers are used as transmitter and receiver alternately to obtain the upstream and downstream ultrasound propagation time sequentially, we proposed one-step mid-air flow rate measurement with a remarkable frame rate through the V-shaped bidirectional beam generated by a 3.6 mm × 3.6 mm 5-channel ~250 kHz PZT PMUT phased array. By utilizing the grating lobe produced through optimized array pitch design and sequential control, this 5-channel PMUT array breaks the conventional design limitations typically associated with grating lobes, and generates the V-shaped beam with dual main lobes measured at 27° and 153°, enabling 1000 times of upstream and downstream ToF measurements in 1 s. Furthermore, installation geometry optimization was proposed to enhance the ToF resolution and adaptation to various pipe circumstances, where flow rate measurements in conventional straight pipe and optimized zigzag-shaped pipe with 150 mm sufficient ultrasound propagation length were investigated. The experiment results demonstrated the superior flow rate monitoring performance of our device and system, where large-range (0.5–35 L·min^–1 or 0.045–3.177 m·s^–1, airflow) and high-resolution (185 ns/(L·min^–1) or 2032 ns/(m·s^–1)) flow metering with significant linearity (0.997) was successfully obtained, revealing great potential in advanced flow monitoring application scenarios.</p><p></p>