Abstract: In order to study the influence of guiding cone on radiation symmetry of compressed core, X-ray emission measurements in fast ignition pre-compression were tested on Shenguang Ⅲ prototype facility. Emission images of different targets in hohlraums with the same length and that of the same target in hohlraums with different lengths were recorded. Least-square ellipse fitting was applied to 50% peak intensity contour, thus the ellipse characters could be used to numerically analyze the core compression symmetry at peak emission time. The results validate that the compression symmetry is reduced, as the emission region is stretched in equatorial direction, if the target is attached to a gold cone. The compression symmetry can be adjusted by hohlraum length, as the shape deformations of cone-guide target in hohlraum with 1600 m length is less than that in hohlraums with 1500 m and 1700 m length.