A facial method for preparing reverse osmosis polyamide (PA) membranes of excellent antifouling and separation performance was developed via covalently grafting phosphonic acid on membrane surface. First, a pristine PA layer was synthesized by interfacial polymerization between m‐phenylenediamine and trimesoyl chloride. Then, a second interfacial reaction was implemented between ethylenediamine and the residual acryl chloride on the pristine PA layer, generating an active layer enriched by primary amine. Finally, the amine‐rich surface treated by formaldehyde and phosphorous acid to produce a membrane surface modified by phosphonic acid groups. Surface characterization by attenuated total reflectance infrared, X‐ray photoelectron spectroscopy and zeta‐potential measurements illustrated the presence of phosphonic acid group. The lowest contact angle of modified membrane was 26°, demonstrating the membrane possessed an outstandingly wettable surface. The optimal separation performance was 88 L m⁻² h⁻¹ of water flux and 99.4% of salt rejection under 1.55 MPa. In addition, bovine serum albumin was used as organic foulant to measure the antifouling property of membranes. The result of dynamic fouling experiments indicated that the modified membrane exhibited better antifouling (of which the irreversible fouling degree was 7.1%) compared with commercial membrane BW30 (of which the irreversible fouling degree was 13.5%). © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 46931.