The South China Sea holds abundant oil and gas resources, yet deepwater low-temperature conditions retard cement strength development, increasing cementing costs. In response, this study incorporates rice husk ash (RHA)—a green construction material—into oil well cement to systematically evaluate the effects of RHA dosage (5%, 10%, 15%) and particle size (11.4–56.9 μm) on early-age hydration properties. Compressive strength tests and isothermal calorimetry were used to characterize mechanical performance and heat release behavior. Thermogravimetric analysis (TG-DTG) and scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS) were employed to clarify the regulation mechanism of RHA size and dosage on early cement performance and to reveal the enhancement mechanism of RHA. Results show that RHA dosage and particle size jointly regulate cement hydration, microstructure evolution, and hydration product formation, significantly affecting mechanical properties. At 1 day, cement with 5% RHA exhibited the highest strength; at 3 and 7 days, cement with 10% RHA showed maximum strength. As RHA particle size decreased, 1- and 3-day strength gradually increased, while 7-day strength first rose and then declined, peaking in the T1RHA group. Low RHA dosage enhances strength through pozzolanic reaction, nucleation site provision, and space-filling effects. High dosage leads to performance decline due to dilution and particle agglomeration. Finer RHA particles show higher pozzolanic activity and more pronounced exothermic reaction. However, excessively fine particles can inhibit later hydration. This study provides a theoretical basis and technical reference for optimizing RHA application in oil well cement.