推导了涡旋光束传输一段距离后观测平面上电场的解析表达式，在此基础上对观测平面上的光强分布及光斑大小进行了分析。和传统的高斯光束不同，涡旋光束传输后的光斑尺寸没有较为统一的定义。提出了均方根光斑尺寸以及等效光斑尺寸的定义，并分别采用这两种定义对涡旋光束的光斑展宽进行了分析。研究表明，涡旋光束在传输中的光斑展宽由拓扑电荷数以及传输距离决定，拓扑电荷数越大，光斑展宽越快。数值计算进一步说明，等效光斑尺寸更适合用来表征涡旋光束传输后的光斑大小。

The analytical expression of the electric field is derived on the observation plane for vortex beam after propagation. Based on the derived result, the intensity distribution and the beam size on the observation plane is analyzed. Different from the traditional Gaussian beam, there is no generous definition for vortex beam after propagation. The mean-square width and the equivalent-beam width are put forward to investigate the spreading of vortex beam while propagating. It is shown that the spreading of vortex beam depends on the topological charge and propagation length. The beam with bigger topological charge spreads faster. Calculation results reveal that the equivalent beam length is more suitable to describe the beam size of vortex beam after propagation.