Precision casting Ti2AlNb alloy is used in aircraft engine components with a high temperature service environment, there may be anomalies, causing overtemperature service damage and affecting the mechanical properties of the material. However, few studies focused on the damage behavior of Ti2AlNb alloys in overtemperature service. In order to better understand the behavior of degradation of the properties of finely cast Ti2AlNb alloys due to the occurrence of service damage during actual service. In this paper, Microstructure degradation was investigated in a precision casting Ti2AlNb alloy after overtemperature treatment at 750°C and 850°C by using an optical microscope (OM), scanning electron microscopy (SEM), and quantitative analysis by using Image J. Also, microhardness changes were studied, and tensile tests were performed at room temperature and 650℃. The results show that the Ti2AlNb alloy consists of the B2 phase and the lamellar O phase before and after overtemperature treatment. After the overtemperature treatment, the B2 phase was partially transformed into the O phase, and the O phase lamellar underwent discontinuous coarsening and the lamellar thickness increased with temperature and time increasing. O-phase content increases gradually as well; The microhardness, tensile properties at room temperature and 650℃ of Ti2AlNb alloys decreases with the increase of overtemperature and time. Of these properties, the high-temperature tensile strength exhibited the greatest decrease. Experimental and statistical analysis shows a linear relationship between microhardness and O phase content. Therefore, the overtemperature service damage of precision casting Ti2AlNb can be evaluated by the microhardness and/or content of O phase.