Small-size at-bit azimuthal gamma geosteering tool is an important tool used in the process of old well sidetracking. It is significant to carry out numerical simulation analysis of erosion and life prediction to reduce development costs and excavate the remaining reservoir in old wells. By using the finite element software, we calculate and analyze the erosion characteristic of the drilling fluid flow and the sand content ratio of the geosteering tool. These results demonstrate that the water hole erosion rate is positively correlated to the flow rate and sand ratio respectively, and the maximal erosion rate all happens at the root of the female thread. When the drilling fluid flow rate is 14 L/s and the sand content ratio is 4%, its erosion life is about 743.2 h which satisfies the service life of downhole tools. The research results can provide theoretical guidance for configuring drilling fluid on-site, adjusting pump pressure, and prolonging the service life of tools.