Using polymer gel is still the mainstream technique of the chemical method for water shut-off in mature oilfield development. The gelation performance of polyethyleneimine (PEI) cross-linking partially hydrolyzed polyacrylamide (HPAM) gel was systematically investigated in this paper by using three types of molecular weight (M-w) HPAM. Results show that the gelant solution (the fluid solution of cross-linker and polymer that exists before gelation) can slightly gel even at room temperature and form a rudimentary 3D network structure for protecting the subsequent gelling. The main factors, including HPAM, PEI concentrations, HPAM M-w and total dissolved solids (TDS) to affect the gelation performance are also discussed. The gelation time is around 18-72 h at 65 degrees C, which is much longer than the commonly used chromium(III)-acetate cross-linking HPAM gel system. The gelation performance of the PEI/HPAM gel system is improved after flowing through porous media and shows the adverse law for a porous media sheared chromium(III)-acetate or phenol-formaldehyde cross-linking HPAM gel system. Atomic force microscopy (AFM) scanning results show that the microstructure of PEI/HPAM gel distributes a lot of cavities. Compared to the original premature gel (code B), the cavities are more uniform and regular after flowing through porous media. In addition, this study revealed that the presence of free oxygen in the uniform cavities of the weak crosslinked gel after flowing through porous media can accelerate the oxidation reaction to produce a darker brown gel. It also discusses how these new findings will affect the application of the gel system in the oil field.