The phenomenon observed by Wu et al., which takes place in the interation of two nonpropagating solitary waves in a trough resonator, is analytically analyed by the perturba-tive variational analysis. The coalescence period and the first-coalescence-time for in-phase solitary waves as well as the interaction forces and the interaction potential existing between two solitons are given, providing a partical description for the interaction between two non-propagating solitary waves.