Modern coexistence theory indicated that stable coexistence needs sufficiently strong niche differences to prevent competitive exclusion. However, increasing previous studies suggest that facilitative interaction also plays an important role in community structure, which provides an expanding framework to understanding coexistence. Because rocky intertidal sessile assemblage has vertical gradient, which is a typical system to study how interaction strength varies along vertical gradient. However, the variation of mutual interaction strengths along vertical gradient in intertidal system has not be studied yet. In this study, we elucidate how interaction strength of same species combination varies with vertical gradients and try to explain how facilitative interaction maintains species coexistence. To achieve this goal, a multi-step analysis based on Bayesian parameter inference was used. Changes in coverage of Gloiopeltis furcata and Chthamalus challengeri were monitored for 18 years at 44 plots. Data were analyzed by fitting a multivariate dynamics model to estimate the interaction strength matrix (α_ii, α_ij, α_ji and α_jj) and intrinsic population growth (r_i, r_j). The results show that interspecific interaction (α_ij, α_ji) switch from negative to positive when the environment is harsh. And more detail will be showed in conference.