Based on the respective characteristics of gel conformance control systems of HPAM/Cr³⁺ and HPAM/phenolic resin prepolymer，the chromic acetate and phenolic resin prepolymer（PRP）composite can not only improve the thermal stability of HPAM/Cr³⁺ gel，but also promote the better application of HPAM/PRP gel in low-medium temperature oil reservoirs. Besides，it can improve the mechanical properties of the HPAM gel. However，the mechanism of the composite cross-linking reaction between Cr³⁺ -PRP and HPAM is still unclear. Therefore，by characterizing the performance differences between Cr³⁺ -PRP solution and PRP solution，the chemical reaction process between Cr³⁺ and PRP was studied. Thus，by comparing the reaction differences between Cr³⁺ -PRP/HPAM and Cr³⁺ /HPAM，PRP/HPAM，the mechanism of the composite crosslinking reaction of Cr³⁺ -PRP/HPAM was revealed. In a Cr³⁺ -PRP solution，because the hydroxymethyl and phenolic hydroxyl groups on the benzene ring of PRP are more active than acetate，a new polynuclear hydroxyl bridge complex ion（Cr³⁺ -PRP complex）was formed. The cross-linking reaction between the Cr³⁺ -PRP complex and HPAM was a multiple complex reaction. At first，part of Cr³⁺ from the Cr³⁺ -PRP complex reacted with HPAM，and then the remaining Cr³⁺ and all PRP in the complex reacted with the polymer at the same time. The cross-linking reaction of Cr³⁺ and carboxy group could affect the cross-linking reaction of PRP and amide group. The reaction process of Cr³⁺ -PRP complex with HPAM was different from the superposition of the reaction of Cr³⁺ /HPAM and PRP/HPAM. The experimental results promoted the understanding of the crosslinking mechanism of Cr³⁺ -PRP/HPAM system，which could lay a certain theoretical foundation for its efficient application.