Marine microplastic pollution is one of the most concerned global environmental problems. In the past ten years, marine microplastic research has developed rapidly, but it faces some unresolved key scientific challenges, including the technical breakthroughs in the detection of small-sized microplastics and nanoplastics, the establishment of standardized detection and analysis methods of microplastics and nanoplastics, the optimization of marine microplastic transportation models, the improvement of ecological risk assessment schemes, and etc. This paper provides an overview of the current status of microplastic pollution in offshore, oceanic and polar regions. We summarize the challenges of marine microplastic research in sample collection, detection, and analysis, as well as the existing deficiencies in the marine microplastic stock estimation, transportation simulation, and ecological risk assessment. We also provide suggestions for future microplastic research, aiming to provide references for marine microplastics' routine monitoring, scientific research, risk assessment, and governance.