To explore the roles of the synergistic effects of CO2 curing and Nano-TiO2 (NT) on the mechanical properties of Low-Heat Portland Cement (LHPC), the flexural and compressive strengths of LHPC were tested during various CO2 curing times and NT dosages. The influence mechanisms were analyzed using testing methods such as scanning electron microscopy and thermogravimetric analysis. The results indicate that there is an optimal range for the NT dosage and CO2 curing time when considering only the addition of NT or CO2 curing. During the synergistic effect of CO2 curing and NT, the compressive and flexural strengths of the LHPC pastes cured with CO? within 24 h significantly increased at 14 and 28 d when the NT dosage was 1% to 1.5% and 1.5% to 2%, respectively. This is attributed to the increased masses of C-S-H, AFt, and CaCO3 in the samples, as well as the refinement of pore sizes greater than 50 nm by fibrous C-S-H with a smaller length-to-diameter ratio. Conversely, when CO2 curing exceeded 24 h and the NT dosage was greater than 2%, the mechanical properties were reduced due to an increase in the number of pores, the formation of whisker-like aragonite with larger length-to-diameter ratios, and the generation of micro-cracks in the samples. Based on the comprehensive mechanical strength and economy, it is recommended that the curing time for CO2 is 12 h, and the dosage of NT is 1%. This study provides a reference for improving the early mechanical properties of LHPC.