Ocean serves as a major sink for microplastics, and the accumulation of large amounts of microplastics seriously impacted its ecological environment. However, given the complex physicochemical properties of microplastics, single-parameter characterization methods fall short in comprehensively assessing their ecological effects and toxicological risks. To address this challenge, a microscopic electromagnetic heating pyrolysis-mass spectrometry method (MEhPy-MS) was developed in this study. And four-dimensional parameters of microplastics, including quantity, mass, type, and particle size, were characterized simultaneously using this method. Integrated with an image recognition algorithm, the microscopic imaging module determined about 323 particles in the seawater samples, with sizes ranging from 271 to 325 μm. The EhPy-MS module determined quantified polystyrene (PS) at 3.15 mg and poly(methyl methacrylate) (PMMA) at 1.79 mg. Based on the particle size distribution and density of the microplastics, the number of PS particles was approximately 224 while that of PMMA particles was approximately 115, yielding a total count error less than 5%. By enabling the simultaneous characterization of multiple parameters on a single platform within 8 minutes, this method significantly enhanced both data comparability and analytical throughput. This work provides a powerful tool for the microplastics characterization in marine environments, facilitating their ecological effects and toxicological risks assessment.