Agricultural mulch film (AMF) is deemed an important source of microplastics (MPs) in agricultural soil (AS). However, quantitating the contribution of AMFs to MPs in farmland soil and surface water remains a considerable challenge to date. In the present study, a basic framework was developed to address these concerns. First, the concentrations of MPs in soil derived from AMF abrasion (C_MP) and the total MPs from all sources in AS (C_TMP) were measured. Then, the ratios of C_MP to C_TMP, i.e., the contribution of AMFs to MPs in AS, were calculated. The contribution of AMFs to MPs in surface water via soil erosion was calculated based on C_TMP values, the ratios of C_MP to C_TMP, soil erosion intensities (SEIs), and farmland areas. Furthermore, the potential contribution of soil erosion to MPs in the ocean was estimated. In China, the inventory of MPs in surface AS in 2018 ranged from 4.9 × 10⁶ to 1.0 × 10⁷ tons according to our results. AMFs contributed 10%–30% of the C_TMP with certainties of 60–95%. Assuming that all MPs in AS can be exhaustively transferred to surface water via soil erosion, the national mass transfer amount of MPs (MT_TMP) from AS to surface water reached 1.2 × 10⁵−2.2 × 10⁵ tons (∼2% of the inventory of MPs in the AS of China); the fluxes of MPs into the ocean from AS were 3.4 × 10⁴−6.6 × 10⁴ tons, assuming that all MPs in the AS of coastal provinces enter the ocean. It is likely that AMFs contributed 10%–30% MT_TMP and fluxes of MPs to the ocean according to the ratios of C_MP to C_TMP. Apparently, approximately 30% of the national MT_TMP (i.e., the rate of MP flux to the ocean to MT_TMP) was input to the ocean.