The present invention relates to a forward osmosis-based separation membrane based on a multilayer thin film, using crosslinking between organic monomers, and a preparation method therefore, and in the preparation of the forward osmosis-based separation membrane including a support layer and a selective layer, a middle layer is provided between the support layer and the selective layer so as to prevent a phenomenon in which the selective layer is filled in a pore of the support layer, such that the thickness of a multilayer thin film constituting the selective layer is optimized, and excellent water permeability, salt removal rate and pollution resistance properties are exhibited through the support layer having a structure of uniform surface pores and minimized pore distortion.