Effectiveness of phosphate (P) immobilizing lead (Pb) in soil is often unsatisfied, mainly due to the limiting mass transfer in soil porous system and the kinetics of Pb liberation from active mineral into the soil solution. Insufficient information has achieved on the in-situ pore-scale lead immobilization processes. In this work, we carried out visualized Pb immobilization by 0.5 mol/L monopotassium phosphate in 1D counter-diffusion U-tube, which composed of equivalent amount of crystallized lead carbonate in the 0.2wt% porous agarose hydrogel. Migration of the adding P was solely controlled by counter-diffusion. As illustrated by optical microscopy and SEM images, lead carbonate of homogenous hexagonal rod-like crystals gradually developed significant reaction front and zonation in different horizontal locations, and transformed crystals into scattered elongate large-size platy morphology in the cationic side and dense small needle-clusters in the anionic side until a balance had reached after more than 3 weeks. Occasionally, there were horizontally distributed filamentous or chain-linking crystals, which may be the accretion pathway of tiny crystal particles. Results also revealed that the original surface of protogenetic minerals were etched by the secondary minerals, making the surface with arranged mosaic pattern. Crystal surface become less smooth as the agarose pore size get larger, and debris of unassembled sediments formed when adding humic acid, implying the flexible crystal growth mode with various conditions, thus challenged the mineral stability. Finally, some unconverted lead carbonate minerals had been found in the unreachable middle of the tube due to the compact crystal density, responsible for the potential release risk once the sediments get crack. The above results revealed the in-situ mineral phase transformation during the immobilization processes, providing better understand and feasible improvement approaches of the lead immobilization effectiveness.