The 50 Hz harmonic component distribution along HVDC transmission line when energizing a no-load transformer is related to the harmonic impedance at the inlet of HVDC line. This paper studies the characteristics of harmonic impedance at the inlet of HVDC transmission line in detail. The harmonic impedances in different operation modes are analyzed and compared with each other qualitatively. The analytical equivalence model of the harmonic impedance is studied, and its accuracy is verified by comparing the calculation results of the analytical method and the test signal method in different operation modes based on the simulation model of an actual HVDC system. Furthermore, the influence of the operation mode on the harmonic impedance is analyzed according to the analytical calculation results, and its conclusion is consistent with the above qualitative analysis. The effect of stray capacitance of the converter transformer casing on harmonic impedance at the inlet of the HVDC transmission line is studied by changing the size of stray capacitance in the converter model, and the equivalence model is simplified. This work is supported by Science and Technology Project of Guangdong Power Grid Limited Liability Company (No. GDKJQQ20152039).