Ultrasonic sensors are widely used inmobile robots for spatial perception. To extend the spatial perception range, current mobile robots often adopt a multi-sensor approach, resulting in a sensing system that is large in volume, high in power consumption, and complex in design. In this paper, wepropose a detection method based on a dynamic piezoelectric micromachined ultrasonic transducer (pMUT) that realizes bi-directional detection with a single device, effectively doubling the measurement range while maintaining a compact size and low power consumption. This method leverages the two-way vibration characteristics of the piezoelectric film to capture ultrasonic echoes in both directions, enabling distance measurements in two directions. Results indicate that the one-way measurement range remains unchanged with this method, while distances to obstacles in both directions can be accurately measured, reflecting their corresponding directions under dynamic conditions. This method simplifies the sensing system and is expected to play an important role in obstacle avoidance for measurement.