The invention discloses a method for calculating the maximum shape coefficient of a rotor for a pump. The pump is provided with a rotor, the molded line of the rotor is composed of a peak-shaped linesegment outside the pitch circle and a valley-shaped line segment in the pitch circle, the peak-shaped line segment and the valley-shaped line segment are formed by single line segments or multiple segments, the peak-shaped line segment and the valley-shaped line segment which are connected with each other and located on the segment are correspondingly defined as a peak main line segment and a valley main line segment, the peak main line segment and the valley main line segment form a conjugate rotary motion of the rotor body, and the shape coefficient is defined by the ratio between the peakradius of the rotor and the radius of the pitch circle. When the peak main line segment is in a critical state with the inflection point or the equivalent valley main line segment is in a critical state with no geometric interference, and the curvature radius line at the inflection point of the peak main line segment is divided by the intersection point of the curvature radius line and the pitch circle. The inflection point curvature radius line is perpendicular to the connecting line between the inflection point curvature center and the rotor center.