Mathematical relation model that between parts length of fixed length rolling technology and roller diameter was established based on the established physical model of circle section rolling flat. The roller diameter of fixed length rolling was solved, influence factors of parts length as circle section rolling flat were analyzed, the adjustment and control method in rolling process were put forward, which took the rolling forming of JD entirely discharge electrode wire as an example. Error of part length reaches to 1.16% in the actual rolling case, which meet the technical quality requirements and obtain good application results.