The environmental capacity of agro-ecosystem is the basis of sustainable development of agriculture, but this is hard to evaluate quantitatively due to complex input and output processes of heavy metals. Therefore, in this study, leaching of heavy metals based on PROFILE weathering model were integrated into the steady-state critical load (SSCL) of heavy metals. The results showed that the leaching rates of Hg, As, Cd, Cr, Pb, Cu and Zn in paddy soil were 0.08, 4.69, 0.22, 44.31, 18.13, 21.96 and 64.42 g/ha ‧a, respectively, while the leaching rates were significantly correlated with pH, CaO, TFe2O3 and C_org.. Atmospheric deposition was the main input source of heavy metals in agricultural soil, while rice plant uptake and leaching were the main output pathways. The spatial distribution of SSCL were mainly affected by the content of heavy metals in soil, Aw (specific area of soil mineral), and ρ (bulk density). Values of SSCL hardly changed after about 40 years (Hg≈0.02 kg/ha, As≈0.60 kg/ha, Cd≈0.07 kg/ha, Cr≈ 5.59 kg/ha, Pb≈3.55 kg/ha, Cu≈1.49 kg/ha and Zn≈4.45 kg/ha). However, the sensitivity analysis indicated that soil leaching had 24.30%-27.90% positive effects on SSCL model. Based on the relationship among leaching, pH, standard limit and SSCL of heavy metals, the standard limit could be appropriately raised to cope with the increased human activities on the premise of the ecological capacity. Thus, the SSCL model provides a new insight for the establishment of environment management in agricultural soils.