In this paper,a detailed description for the operation principle,structure and performance of a twin core fiber optical add/drop multiplexer (OADM) was given using the coupled mode theory.The influence of the device parameters on the performance of the twin core fiber OADM was discussed.Result shows that,in order to realize the add and drop function at the same time,the strong fiber grating is preferred to shorten the fiber grating while maintaining its high reflectivity.