Abstract: The colour characteristic of synthetic alexandrite samples are analysed by the ultraviolet-visible spectral analysis. The influence on colour change effect by the colouring elements and their contents are primarily studied. The results indicate that the common violet tone of synthetic alexandrite is related to the incomplete absorption of absorption spectrum in the violet region, which could be weaken by joining a few colouring elements that redound to absorption from the violet region.The absorption band in the orange-yellow region could distribute rationally by increasing the vanadium element which balances the permeation of light between the red region and green region for perfect colour change effect. The UV-VIS spectrum of synthetic alexandrite has the great relationship with the crystallography direction. The absorption spectrum of sample in the direction of (001) has a strong absorption band in the orange-yellow region where the sample has the obvious characteristics of colour change effect.In the directions of (100) and (010), the sample has strong dichroism, but the colour change effect is not obvious.