Based on the background of the domestic ±800 kV UHVDC Power Transmission Project, characteristic harmonics at AC side of converter station is researched by theory analysis and mathematical deduction, respectively, ignoring converter commutation process and considering the converter commutation process. At the same time, characteristic harmonics at DC side of converter station is researched. According to the above research, taking the actual ±800 kV UHVDC Power Transmission Project as example, AC and DC side characteristic harmonics of converter station are calculated by characteristic harmonic load flow algorithm and iteration harmonic algorithm. The result shows iteration harmonic algorithm is feasible for harmonics calculations of UHVDC transmission system.