It is an ideal place to search for exotic states through J/ψ hadronic decays. In this paper, we discuss the way to use the partial wave analysis (PWA) based on covariant helicity amplitude analysis to study the invariant mass spectrum of ηπ and to find the evidence of the existence of exotic states in this channel. The formula for PWA is also given. It is the theoretical foundation of experimental physics analysis on exotic states searching in J/ψ hadronic decays. The theoretical formula can also be used in physics analysis of J/ψ decays into ωππ, φππ, ωKK, φKK and K^*(892)Kπ.