A novel method to measure the laser induced damage threshold (LIDT) of crystals is presented. The method involves confirming the cross-sectional power density distribution of incident laser and measuring crystals' radii of damage spots, then calculating the corresponding LIDT. For Gaussian laser beam, a relational expression between crystals' damage threshold and radius of damage spots has been reasoned out. Different from other measuring method, this new method can qualitatively and exactly measure crystals' laser induced damage threshold. Using the method, the laser induced damage thresholds of LiNbO3 single crystals doped with different MgO concentration are measured under the same laser condition. The measuring results agree well with both former relevant reports and authors′ theoretical analysis. The applicability of the new method is discussed. It is shown that the proposed method is fast, accurate and can be widely used to precisely test crystals' laser induced damage threshold.