A machine vision measurement method for diameters of large rotary parts based on the intersections of laser stripes was proposed to realize on-line and non-contact measurement in industrial field. Combined with geometric constraints, feature information of measurement for diameters was constructed with high-strength and high-directivity lasers. Feature points for measurement were emphasized by projecting intersecting laser stripes on the surfaces of large rotary parts. A high-precision extraction method was proposed based on the distribution character of intersections on the surfaces of large rotary parts. The space coordinates of intersections were reconstructed through calibration results and the diameters can be calculated by modeling the corresponding elliptic. Measurement experiment for diameters was carried out in a laboratory, of which the results show that the measurement error was 0.5%. It provides a reliable technology for on-line and non-contact measurement for large rotary parts.