Bacillus anthracis marker (dipicolinic acid, DPA) is a biological warfare agent. The excessive amounts of the compound would lead to serious problems in human health. It is essential to develop a straightforward and rapid detection technology of DPA. For this reason, an optical sensing platform was established on a robust alginate film. Firstly, green emission CQDs were synthesized via a one-step microwave carbonization method based on citric acid and urea. In the next step, europium ions were employed to chelate the carboxyl groups of CQDs and sodium alginate was selected to assemble the green fluorescent film (CQDs-Eu film). In the presence of DPA, the emission peaks evolved from green to red region. The ratio of I-615/I-554 linearly increased with the addition of DPA in the range of 1-45 mu M with a detection limit as low as 0.17 mu M. This CQDs-Eu film-based sensing nanostructure showed its potential applications in the fields of biology and food analysis.