1. A method of rendering an expanded lumen image, comprising: obtaining a plurality of viewpoints by sampling a centre line of a lumen based on a three-dimensional lumen image; establishing a spherical projection plane for each of the viewpoints, wherein a point on the spherical projection plane corresponds to a point on an inner wall of the lumen; determining a relationship between a two-dimensional projection plane and the inner wall of the lumen according to a corresponding relationship between the spherical projection plane and the two-dimensional projection plane; and obtaining a two-dimensional expanded image of the three-dimensional lumen image by performing image rendering on the two-dimensional projection plane according to the determined relationship between the two-dimensional projection plane and the inner wall of the lumen, wherein establishing a spherical projection plane for each of the viewpoints comprises: determining a gazing direction and a local coordinate system of the viewpoint, wherein a Z-axis direction of the local coordinate system is the gazing direction of the viewpoint; and establishing the spherical projection plane by taking the viewpoint as a centre of sphere and R as a radius under the local coordinate system, wherein R is a preset radius value less than or equal to a radius of the inner wall of the lumen.