Parity-time (PT) symmetry, as a physical concept, describes the special and temporal symmetry of the physical quantity. Recent year, it has been discovered that PT symmetry can be easily observed in the photonics system. Exceptional point (EP) is a particular point in the PT symmetric system, at which the eigenvalues and eigenvectors degenerate. Plenty of counterintuitive phenomena have been observed around EP. Specially, owing to the non-Hermiticity induced nonadiabatic transitions, chiral mode switching was achieved by encircling the exceptional point (EEP) in a PT symmetric system.