To study the impact of ultra high-voltage direct current (UHVDC) hierarchical connection mode on power system,the calculation methods of hierarchical interaction factor (HIF) are proposed and the causes of the deviation between different methods are analyzed.The impacts of UHVDC hierarchical connection mode on HIF,effective short circuit ratio (ESCR) and transient over-voltage (TOV) are investigated compared with the traditional connection mode.The results indicate that the HIF is smaller,the ESCR is larger and the TOV is smaller under the hierarchical connection mode.The hierarchical connection mode is more superior with the traditional one.