The mechanism of the cross-linking reaction between hydrolyzed polyacrylamide (HPAM) and chromium acetate in formation water was systematically studied by viscometry, ultraviolet-visual absorption spectrometry, and core flow experiments. The results show that the process and outcome of the cross-linking reaction between HPAM and chromium acetate is significantly affected by salinity. In formation water, chromium acetate cannot cross-link HPAM when the salinity is too low. With an increase of salinity, the cross-linking reaction occurs, and the degree of the reaction increases to a balance point. When the cross-linking reaction occurs, two different types of the reaction appear under different salinity. The intramolecular cross-linking reaction is the first to happen, whether under high salinity or under low salinity. In this stage, the cross-linking reaction is a first-order reaction and plays a greater role in the whole process of the reaction due to its higher reaction rate and higher reaction degree. Next, the intramolecular cross-linking continues to occur if the formation water is in the low salinity range, but the intermolecular cross-linking reaction occurs if the formation water is in the high salinity range. In this stage, the cross-linking reaction is a multistage reaction. Meanwhile, the experiments quantify a matching relationship between cations in the formation water and the profile control and oil displacement agent. The experimental results in this paper can provide theoretical guidance for the practical application of profile control and oil displacement using HPAM/Ac3Cr.