1. A method of auditory evoked heart rate variability analysis for evaluating function of autonomic nervous system, comprising the following steps of: capturing a first electrocardiogram signal of a person with a first time period without any sound; stimulating the auditory system of the person by a sound with the intensity of the sound within 30 dB to 100 dB with a second time period through a sound transmission device; capturing a second electrocardiogram signal of the person under the sound; performing an analog-to-digital conversion of the first and the second electrocardiogram signals; selecting the peaks of the first and the second electrocardiogram signals after the analog-to-digital conversion and transforming peaks of the electrocardiogram signal to a first heartbeat cycle and a second heartbeat cycle curve, respectively; performing a spectrum analysis of the first heartbeat cycle and the second heartbeat cycle curve to obtain a first spectral parameter and a second spectral parameter; and obtaining a difference between the first spectral parameter and the second spectral parameter to obtain a change index of function of autonomic nervous system.