Abstract: A quantitative analysis method for six principal active constituents (acubin, geniposidic acid, chlorogenic acid, pinoresinol di-O-glucopyranoside, geniposide, and pinoresinol 4-O-glucopyranoside) of crude Eucommiae Cortex (EC) and its salt-processed product extracts was developed to investigate and compare their pharmacokinetic behaviors in adenine-induced renal fibrotic rats in vivo. UHPLC-QqQ-MS/MS technology was employed. Scan was conducted in negative ion mode and quantitative determination was carried out by MRM paired ion. The established method was fully validated by specificity, linearity, precision, accuracy, stability, recovery, and matrix effect, and the results of methodological investigation met the requirements of biological sample analysis. Then, a quick, sensitive, and accurate method was successfully established, which could simultaneously measure the contents of six active constituents of crude and salt-processed EC extracts in rat plasma. After a single administration to renal fibrotic rats of crude EC and its salt-processed product extracts, the plasma concentration of each constituent at different time points was measured, the pharmacokinetic parameters were calculated and the concentration time curves were structured. The experiment was approved by the experimental animal ethics committee from Nanjing University of Chinese Medicine (No. 202103A008). The results showed that compared to the crude Eucommiae Cortex group, the t_max of aucubin, pinoresinol di-O-glucopyranoside, geniposide, and pinoresinol 4-O-glucopyranoside in the salt-processed Eucommiae Cortex group rat plasma were significantly lower than those in the crude group (P < 0.05, P < 0.01); the C_max and AUC_{0-48 h} of chlorogenic acid, the C_max, AUC_{0-48 h} and AUC_0-∞ of pinoresinol di-O-glucopyranoside, and the C_max of geniposide and pinoresinol 4-O-glucopyranoside were significantly higher than those in the crude group (P < 0.05, P < 0.01). Our investigation found that compared to crude Eucommiae Cortex, a variety of active ingredients could play a role of quick effect with higher peak blood concentration and bioavailability after oral administration of salt-processed Eucommiae Cortex, which were consistent with the traditional Chinese medicine theory of "salt-processing enhancing drug into kidney meridian", providing an experimental basis for the selection of quality control indexes and the in-depth study of processing mechanisms and metabolic rules in vivo of Eucommiae Cortex and its salt-processed product.