The energy fluctuation of the beam which is produced by the large scale electron linear accelerator (linac) is statistically analyzed. The result shows that the mutual contribution of disturbances exists in this linac. We took account of these effect of disturbances, and we made the energy fluctuation model by the adaptive identification. As a result, the energy fluctuation was surpressed less than ±0.1% by the control based on this model. The statistical analysis and the adaptive identification will be indispensable for the next generation of linac like a free electron laser.