A circuit board and a circuit design method are provided. A first shielded line is disposed between two parallel differential lines. The first shielded is an electric line in parallel with the differential lines and disposed on the symmetrical line of the two differential lines. Also a second shielded line and a third shielded line which are parallel each other are added. The second shielded line is symmetrical with the third shielded line relatively to the symmetric axis of the differential lines. A fourth shielded line is added, which is in vertical connection with the first shielded line, the second shielded line and the third shielded line. The fourth shielded line is symmetrical with the symmetric line of the two differential lines. External radiation of the differential lines is reduced through the shielded line structure.