Measurement and correction of coupling in BEPC II

The existing linear coupling theory and representation method are introduced briefly. The socalled local and global coupling is discussed in more detail. The vertical orbit distortion excited by a horizontal corrector is represented with the coupling parameters at the corrector and the observation point. The formula is used to measure the coupling in BEPC II. In order to correct the coupling, vertical correctors are used to change the vertical orbit through sextupoles by a least square method. We also introduce and review other frequently used coupling measurement/tuning methods used in our machine.