A lung sound monitoring device is provided. The lung sound monitoring device includes an acoustic sensor and a processor. The acoustic sensor is configured to capture the chest cavity sound of a subject at a first monitoring position on the subject and convert the chest cavity sound into a first chest cavity sound signal. The processor is configured to receive the first chest cavity sound signal and perform a filter process to obtain a first lung sound signal, and convert the first lung sound signal into a first lung sound spectrum using time-domain frequency-domain conversion. The processor acquires a first intensity index according to the first lung sound spectrum, and outputs a prompt signal according to the first intensity index to indicate whether the first monitoring position is a qualified monitoring position.