The oil-rich RAP fine material has a high proportion and significant asphalt content, offering considerable utilization value. However, in conventional production processes, it faces challenges such as difficult heating and insufficient activation of aged asphalt. To achieve deep regeneration, this study adopted the method of preparing recycled asphalt mortar to regenerate asphalt mixtures. However, there is currently a lack of quantitative evaluation methods for the activation degree of aged asphalt in this mortar regeneration process. Therefore, it is essential to establish an activation degree evaluation index to optimize the mortar preparation process parameters. The paper calibrated recycled asphalt mortar samples with different activation degrees, studied the variation of the comprehensive performance of the recycled asphalt mortar with the dissolution time, and proposed a quantitative evaluation index for the activation degree of aged asphalt in RAP fines based on the sensitivity of different performance indicators of the recycled mortar to the activation degree. The results show that extending the dissolution time of the recycled asphalt mortar can effectively improve the mechanical properties, rheological properties, and low-temperature deformation resistance of the material. The performance improvements primarily result from enhanced tensile deformation capacity and fracture energy, rather than an increase in peak stress, thus significantly improving the fracture toughness of the mortar. Under the mortar-making conditions of indoor mechanical stirring at 160 ℃, the RAP activation degree corresponding to different performance indicators increases with the extension of dissolution time and tends to stabilize after 15 min, indicating that there is a limit value for the dissolution and integration of aged asphalt under this process condition. Based on nonlinear regression analysis, considering both fitting accuracy and sensitivity, the fitting R2 of the low-temperature coefficient is 0.998, with a maximum slope of 5.03. Therefore, the low-temperature coefficient index is determined to be the optimal indicator for the activation degree.