Abstract: The microstructure of contaminated remolded clay was observed using scanning electron microscopy (SEM) after the preparation for the clay contaminated by sodium carbonate (Na2CO3 ) of different concentrations through immersion test． Subsequently，the apparent porosity and pore size distribution of the contaminated remolded clay were extracted by Matlab． Furthermore，the microstructure transformation mechanism of the contaminated remolded clay was investigated． The studies showed that the microstructure of the clay was transformed such as the destruction of flocculent structure，the redistribution of particles and pore，as well as the obvious reduction of pore porosity owing to the contamination and erosion of Na2CO3 ． Based on manual selection of the best threshold level，the apparent porosity of contaminated remolded clay extracted by Matlab was significantly decreased with the increase of Na2CO3 erosion degree． Meanwhile，the apparent pore size distribution was characterized by the gradual evolution from a majority of macro and meso pores to minor and micro ones． Finally，the transformation of the clay microstructure maybe caused by the diverse effects of Na2CO3 : the ion exchange and adsorption with clay particles， the chemical reaction with non-alkali metal cations and free oxide，as well as the neutralization with humic acid．