The piezoelectric micro-machined ultrasonic transducer (PMUT) is an important sensor in the field of ultrasonic ranging. Moreover, enhancing the ranging performance of PMUT is significant for promoting its engineering applications. This study is based on the sound absorption and amplification characteristics of the Helmholtz resonant cavity and describes the design of a cavity structure that reduces sound wave absorption at the back of the PMUT, as well as a horn structure for forward sound amplification. Experimental results show that at a distance of 2 m, the PMUT integrated with the Helmholtz resonant cavity exhibits a significant increase in ultrasonic signal amplitude, reaching 321.13 mV, which is an increase of 90.5 mV (39.2%), compared with the conventional patch film cavity structure. Furthermore, the PMUT equipped with a horn featuring Helmholtz resonance characteristics measured a signal amplitude of 540.72 mV at the same distance, representing an increase of 162.41 mV (42.9%) compared with the PMUT without the horn. These results indicate that combining the PMUT with the acoustic characteristics of the Helmholtz resonant cavity is of great significance for enhancing the application of PMUT in the field of ultrasonic ranging.