在对包层模进行远离截止近似下，利用耦合模理论分析了长周期光纤光栅中导模与包层模的耦合，并与短周期光纤光栅进行了比较，指出长周期光纤光栅的工作特性可由它的相位匹配条件来说明．计算结果与实验现象相符． Coupling mode theory is used to analyze the coupling between guided mode and cladding mode in the long period fiber grating. Compared with the short period fiber grating, it is pointed out that the operating characteristics of the long period fiber grating can be controlled by Its phase matching conditions to illustrate. The calculated results are consistent with the experimental results.