It is important and necessary to effectively detect the chemical warfare agents, such as highly toxic never agent sarin. However, based on the surface-enhanced Raman scattering (SERS) effect, detection of nerve agent simulant dimethyl methylphosphonate (DMMP) which is weakly interacted with SERS-active substrate has been the most challenge for the routine SERS detection method. To overcome this challenge, we put forward a thin water film confined SERS strategy. Under the space-confinement of water film, Raman measurements are carried out in the water evaporation process. The subsequent water evaporation induces concentrating of the DMMP molecules, which are thus successfully restricted within the strong electromagnetic field enhanced area above the SERS substrates, leading to the enhancement of their Raman signals. This study provides a new way to achieve the efficient SERS-based detection of the target molecules weakly interacted with the metal substrates. (C) 2015 Elsevier B.V. All rights reserved.