Abstract: A method for making a protective device for guardrails comprises the steps of preparing a sheet (2) of flexible material; wrapping the sheet (2) of flexible material between at least one portion (3) of a longitudinal rail (6) of a guardrail (5) and at least one portion (4) of a lower longitudinal element (7), positioned between at least two guardrail (5) uprights (8) at a predetermined height, other than zero, from the ground, to form a smooth and continuous surface (11) which at least partly covers the guardrail (5). A protective device for guardrails comprises at least one flexible sheet (2) which can be wrapped between at least one portion (3) of a rail (6) of a guardrail (5) and at least one portion (4) of a lower longitudinal element (7), positioned between at least two guardrail (5) uprights (8), to form a smooth and continuous surface (11) covering the guardrail (5).