A triple-quadrupole liquid chromatography-tandem mass spectrometry (TQ LC-MS/MS) method was established to determine the contents of 14 phenolic compounds in Chaenomeles fructus grown in different regions. Its antioxidant activity was subsequently assessed by measuring the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging capacity, 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS+) radical scavenging capacity, and ferric reducing antioxidant power (FRAP). The content-effect relationships between the contents of various phenolic compounds and antioxidant activity were analyzed by grey relational analysis to explore the substance basis of the antioxidant activity of Chaenomeles fructus. The results indicated that the chlorogenic acid content was the highest in all 18 types of samples of Chaenomeles fructus grown in different regions, ranging from 1 479.68 to 4 902.39 μg/g. A total phenolic content (TPC) of up to 37.13 mg GAE/g was detected in the plant extracts. The DPPH radical scavenging capacities, ABTS+ radical scavenging capacities, and FRAPs of the samples were in the ranges of 10.13%~74.66%, 26.37%~79.44%, and 43.56~132.53 mg TE/g, respectively. The correlation coefficient between TPC and antioxidant activity was>0.91. An analysis of the contenteffect relationships revealed that eight phenolic compounds, including chlorogenic acid, protocatechuic acid, catechin, and neochlorogenic acid, contributed most to antioxidant activity. The TQ LC-MS/MS method developed in this study enabled accurate determination of the contents of 14 phenolic compounds in Chaenomeles fructus grown in different regions. Through correlation analysis, the primary active components contributing to antioxidant activity were determined, and the substance basis of the antioxidant activity of Chaenomeles fructus was elucidated. The results of this study can serve as a scientific reference for quality control and resource development of Chaenomeles fructus.