This invention provides a system for estimating change of status of one or a plurality of particle beams, the system comprises a plurality of particle detectors and an estimating unit, wherein one or a plurality of particle beams is being projected to a substrate. The particle detectors detect the one or the plurality particle beams reflected from the substrate to generate one or a plurality of detected signals. The estimating unit estimates change of the status of the one or the plurality of particle beams according to the one or the plurality of detected signals. By such arrangement and estimating method, the system could estimate multiple beams and achieve beam placement accuracy.