The invention discloses a method which is used for processing a semi-built-up crank main journal basis circle and can measure error automatically, besides, the method can improve processing precision and reduce defective index. From process of a second main journal on, the process is carried out according to the steps as follows: diameter values of a plurality of points and the corresponding angle values are automatically measured in a rotation period of the main journal at the adjacent edge of the main journal to be processed, and roundness error and positioning error of each measured point are calculated; a polar coordinate system is established, wherein, the rotation centre of a machine tool is taken as the origin of coordinates and the 0-degree phase direction of the crank is taken asthe 0 degree of the coordinates; a contour curve of the main journal at the adjacent edge of the main journal to be processed is then drawn according to the roundness error, the positioning error andthe corresponding angle value of each measured point; a standard basis circle curve and the contour curve of the main journal at the adjacent edge of the main journal to be processed are overlapped and compared in the same coordinate system, so as to carry out reshaping process on the main journal to be processed; and the steps a-c are repeated until a qualified basis circle is processed.