Imaging and Fourier Transforming properties of GRIN-rod Lens are discussed firstly.Then the optical spot of GRIN-rod lens is calculated in theory.Taking no account of aberrations,the optical spot size of the lens whose diameter is 1.80 mm is 1.2 μm.Compare the measurement value which is 1.5 μm with the theoretical one,they are almost identical with each others.It also proves the theoretical analysis is reasoned.