To calculate harmonics of a HVDC system， a new software， which is based on the unified fundamental and harmonic frequency load flow algorithm， is developed by VC++ and Delphi. With this software， several important issues of a HVDC system， such as characteristic and non-characteristic harmonics of the 12-pulse converters， the fundamental frequency load flow and harmonic load flow of DC transmission line and AC system， and the system harmonic distortion factor and harmonic loss， can be analyzed simultaneously. Furthermore， it can also calculate the harmonic parameter of DC transmission line according to the structural parameter of DC transmission line and overhead ground wires. In this software， various functions are also provided for data input， edit， maintenance and query. By these functions， type and parameter of AC/DC filter can be adjusted expediently. This software is used on Chinese southern grid. The calculation result of Yunnan-Guangdong±800 kV HVDC system is compared with EMTDC simulation result. The results show that the proposed software is more efficient， accurate and convenient.