GDOP is an important index to measure the precision of the positioning system. The ray tracing-based localization algorithm uses the mirror station to locate the target. In view of the problem that the existed GDOP calculation methods do not consider the correlation between the image base stations (I-BS) and hence cannot analyze the relationship between the accuracy and the I-BS layout of the ray tracing based positioning system, this paper proposes a GDOP calculation method for the ray tracing based AOA positioning algorithm (AOA-RT). In the micro-cell AOA location model, according to the relationship between the I-BSs and the base station, the correlation matrix of position errors of I-BSs is obtained, and the calculation formula of the GDOP is derived. The simulation proves that the GDOP calculation method is reasonable, and we get several related conclusions.