The widespread application of sewage sludge produced from wastewater treatment plants for agricultural use has been regarded as a primary source of microplastics (MPs) into soils. However, little is known regarding MPs in sludge-based fertilizers and their relevant fate in soils as being applied in agriculture. We comprehensively investigated the abundance, polymer size, type, and morphology of MPs in dewatered sludge, sewage sludge composts, sludge-based fertilizer-amended soils, and earthworms by stereoscopy and micro Fourier transform infrared (μ-FTIR) spectrometry methods. The results clearly showed that the quantity of MPs in soils exhibited a close correlation with the application rate of sludge-based fertilizers. The total abundances of MPs were 545.9 and 87.6 items/kg in soils after annual amendment with 30 (field A) and 15 t/ha (field B) of sludge composts, which is significantly higher than that without compost application (field C, 5.0 items/kg). Correspondingly, MPs were found in earthworms with low quantities of 1.8 and 0.4 items/individual in fields A and B, respectively, while no MP was detected in field C. We speculate that sludge composts may act as a vehicle of MPs into soils and then enter soil biota and in turn influence the spread of MPs in the environment.