Aiming at the problems that the laser spot drift is affected by many kinds of interference factors, and the measurement accuracy is difficult to estimate, the method of spot drift characteristic analysis based on Bayesian inference is put forward. On the basis of strict Bayes theorem deduction, the analysis of laser fundamental mode characteristics and the influence of measurement noise on ideal spot drift, the thorough mathematical model of the spot drift characteristics based on Bayesian inference is constructed and the parameters are determined strictly. The simulation results and the theoretical analysis are in good agreement. The method can be used to evaluate the influence of the environment, such as thermal effect, environmental vibration and air disturbance, on the laser drift.