The change of quantities and categories of plankton is essential for studying marine ecosystems. For decades development, there has been an accelerating advancement of plankton imaging system. To detect plankton accurately and effectively, the In situ Dark-field Microscope (OUC-IDM) was designed based on the principle of dark-field microscopy, with the purpose of detecting the plankton and macroscopic particles at the range of 100 μm to 10mm. The OUC-IDM is in a position to non-invasively imaging plankton in-situ at nearly micrometer resolution. It is worthwhile to mention that the design of OUC-IDM is more compact, smaller, and lighter than its predecessor, not only that, both cameras and embedded computers have been upgraded. Furthermore, Full-color information of plankton can be obtained by using a high-power white color LED as light source, which is favorable for identification of plankton in the advanced stage. To validate the usefulness of the OUC-IDM, the instrument was deployed along the coast in Qingdao several times, colorful and vividly in-situ images have been taken and directly used without or with sample image post-processing. Have regard to the functional differences, the OUC-IDM is best employed to complement rather than replace traditional studying.