AbstractTo investigate the compaction characteristics of a single-component polyurethane (PU) mixture with dense gradation, the effects of temperature, humidity, catalyst content, and standing time on the compaction energy index (CEI) of the mixture were analyzed in this study. The air void ratio and splitting tensile strength of the polyurethane mixture at 15°C under different compaction times were also tested. Based on the test results, the effects of compaction time on the volume and strength characteristics of the polyurethane mixture were analyzed. The results showed that temperature, humidity, catalyst content, and standing time have significant positive correlations with the CEI, where standing time has the strongest correlation, followed by catalyst content, temperature, and humidity, respectively. A multiple linear regression model relating the CEI and the aforementioned factors was established. Compaction time, as characterized by the CEI, significantly influenced the air void ratio and splitting strength of the polyurethane mixture after curing. If the compaction of the polyurethane mixture is premature or delayed, the void ratio of the mixture increases after curing and the splitting strength decreases. Therefore, an optimal compaction time exists for the single-component polyurethane mixture. At this compaction time, the CEI of the mixture is approximately 600–800. During the process of single-component polyurethane curing, the increase in the adhesive force of polyurethane and the damage to the structure of the mixture, caused by the released CO2, served as a pair of mutually restricting effects, resulting in different volume characteristics and splitting strengths of the polyurethane mixture under different compaction times.