Methods and devices for controlling movement of a carriage of a multi-leaf collimator are provided. In one aspect, a method includes obtaining a desired position of each of a set of leaves on the carriage in each of a plurality of segments from a field, determining an allowable moving range set of the carriage according to the desired position, the allowable moving range set including a respective allowable moving range of the carriage in each of the segments, determining a respective position of the carriage in each of the segments according to the allowable moving range set, and controlling the movement of the carriage according to the determined positions of the carriage in the segments.