An accelerator system is provided. According to an example, the accelerator system includes a ray source, a multi-leaf collimator including leaves, a multi-leaf collimator controller and a leaf position determining device. The multi-leaf collimator controller is configured to control each of the leaves to move according to a predetermined position. The leaf position determining device is configured to obtain a three-dimensional image of the multi-leaf collimator, determine a sub-field shape and a sub-field size of the multi-leaf collimator according to the three-dimensional image, determine an actual position of each of the leaves according to the sub-field shape and the sub-field size and obtain an error value for each of the leaves by comparing the actual position with the predetermined position for each of the leaves. In this way, the error value for each of the leaves may be used to control operation of the accelerator system.
