The present invention relates to a new type composite freshness-retaining agent for marine products of fish and shrimp, etc. Its effective components include extract of plant seed-coat, sorbic acid or its salt and sodium polyphosphate. The solution prepared with said fresh-keeping agent possesses several functions of inhibiting bacteria, resisting oxidation and resisting browing, etc.. Said invention utilizes the sodium alginate to make film-covering treatment of fish and shrimp, it can effectively prevent the fresh-keeping from losing, reduce bacterial contamination, reduce air oxidation andretain water content so as to make freshness-retaining effect more ideal. It can prolong the freshness-retaining period of fish and shrimp by 3-5 times.