According to the multi-beam interference principle, the prism-leaky waveguide coupler was analysed theoretically, and compared with the prism-waveguide coupler. Oriterions for properly using those two kinds of couplers were also given. Analysis results show that the prism-leaky waveguide coupler can be used for measuring parameters of the thin films and input-output coupling of the optical waveguide as operated substantially under strong coupling conditions. For weak coupling, our results agreed with Tien and Ulrioh's theoretical analyses.