This article reflects the incomplete interpretation of the French Symbolist poetics that the English free verse theories provided, and investigates the important influence that Post-Symbolist poetics (the New Generation poetics) exerted upon an English modernist, F. S. Flint. With Flint as the medium, this article also examines how poets — Ezra Pound, Richard Aldington, Amy Lowell — accepted and modified the French resources. French doctrines, like “Cadence” and the “Constante rythmique”, inspired Flint’s “Unrhymed Cadence” conception, by which some other British-American modernist poets were impelled to a great extent to seek for free verse’s formal property. This article provides a deeper understanding of the complex relationship between British-American free verse theories and French free verse theories.