Amber is a kind of fossil resins, which formed through sedimentation and fossilization of resins secreted by ancient gymnosperm and angiosperm, for millions or even tens of millions of years. According to the chemical composition of fossil resins, amber is classified into five classes: ClassⅠ, Class Ⅱ, Class Ⅲ, Class Ⅳ and Class Ⅴ. Among them, class Ⅰ amber is a kind of fossil terpenoid resins containing labdanoid diterpenoids, and the semi-fossil resins in the formation process are classified as class Ⅰ Copal. Due to the active functional groups such as carbon-carbon double bonds, hydroxyl groups and carboxyl groups containing in labdanoid diterpenoids, during the long process of sedimentation and fossilization, labdanoid diterpenoids in the terpenoid resins were proceeding a series of chemical reactions such as polymerization reaction, cross-linking reaction, esterification reaction and isomerization reaction, and gradually formed into semi-fossil and fossil resins with different degree of polymerization and cross-linking, for instance Copal as semi-fossil resins with inadequate fossilization, amber as fossil resins with adequate fossilization, hard amber as fossil resins with thorough fossilization. In this study, the semi-fossil resin and fossil resin of the three fossilization stages mentioned above were taken as research objects. By means of infrared spectroscopy analysis technology, the changes of absorption peaks of the organic functional groups ware tracked, such as carbon-carbon double bonds, ester groups and carboxyl groups, and the stage characteristics of semi-fossil resin and fossil resin ware verified, and the chemical process of class Ⅰ amber formation was deduced. Through the analysis and comparison of infrared test results, the main research conclusions are as follows: (1) Due to inadequate fossilization, class Ⅰ copal is dominated by the unstable structure with a large number of carbon-carbon double bonds and carboxyl groups, because of the insufficient polymerization reaction, cross-linking reaction, isomerization reaction and deficient esterification reaction; (2) Due to adequate fossilization, class Ⅰ amber is dominated by the stable structure with a few carbon-carbon double bonds and carboxyl groups, because of sufficient polymerization reaction, cross-linking reaction, and isomerization reaction, but the esterification reaction was in the process; (3) Due to thorough fossilization, class Ⅰ hard amber is dominated by a more stable structure without active functional groups, because of the thorough polymerization reaction, cross-linking reaction, esterification reaction and isomerization reaction.