In this work, the effects of weak magnetic field (WMF) on Fe-0 particles were detailedly investigated and the pre-magnetized Fe-0-Fenton process was applied to the advanced treatment of citric acids wastewater. The results showed that with WMF pretreatment, the COD removal improved by 38.4%, and the H2O2 consumption reduced by 27.5%. As for the optimal Fe-0 pre-magnetization conditions, the time was 30 min and the magnetic field intensity was 200 mT. Pre-magnetization had different impacts on Fe-0 with different sizes, only the mu m scale Fe-0 exhibited the promotion in wastewater treatment. The pre-magnetized Fe-0 particles showed good magnetism stability in over 12 h. VSM, BET, SEM, XPS were used to examine the characteristics of the Fe-0; results showed that the Fe-0 particles had good stability of magnetic property and different size Fe-0 had different surface topography. For the first time, it was found that the magnetic memory impacted the energy required for the escape of electrons in depth of the surface iron atoms and resulted in decrease of background intensity in high binding energy. The potential mechanisms of WMF pre-magnetization of Fe-0 and application in Fe-0-H2O2 reaction were proposed, including the magnetization processing of Fe-0 and the inequality reaction on pre-magnetized Fe-0 particles in the Fenton process.