Heart rate variability (HRV) refers to small variations in consecutive normal cardiac cycles and is an important indicator of autonomic regulation. Although the frequency domain indices of HRV, such as low frequency (LF), high frequency (HF) and LF/HF, have been widely used to assess the activity of the sympathetic and parasympathetic nervous systems and their mutual regulation, their validity as indicators of autonomic nervous system function has been questioned. This study analyses the different views on the significance of LF, HF and LF/HF and examines their validity and effectiveness as indicators of autonomic nervous system function. In this study, it is proposed that LF, while influenced by sympathetic nerve activity, is not recommended for use alone to reflect sympathetic nerve activity, as it is also affected by other factors such as vagal nerve activity. HF, primarily controlled by cardiac vagal nerves, accurately reflects parasympathetic nervous system activity. LF/HF can reflect the regulatory role of the autonomic nervous system to a certain extent, but due to the non-linear relationship and the influence of other factors, the index of LF/HF is not precise.