In the process of monocrystalline silicon processing,femtosecond laser is accompanied by plasma-derived luminescence,which is shown in the form of laser spot.Laser ablation effect is affected by laser pulse energy,optical setting,material properties and environmental parameters.The selection of parameters is the key to realize the ideal machining,so the paper introduces the extraction of the centroid of the spot and studies the variation rule of the spot to improve the machining precision.Commonly used centroid extraction algorithm are gray mass center method,gauss curved surface fitting and so on.However,due to the influence of noise,the light gray peak deviations may occur,and the extracted center of mass has a larger error compared with the real center of mass,thus introducing the multivariate normal distribution and fitting Gaussian surface in multiple directions and estimating centroid by maximum likelihood method,the accuracy is greatly improved.