This study investigated the variation of three-dimensional geometric distortion of geometric distortion caused by B0 inhomogeneity and gradient nonlinearity throughout a 6-month period of a 1.5T 700mm-wide bore MR-simulator. A large customized geometric accuracy phantom was constructed and a program was developed for distortion quantification. Experiment results showed that the distortion varied throughout the testing period. Regular validation of geometric accuracy might be needed if prior distortion information is used for distortion correction. Nonetheless, the variation of distortion had minimal effects on the sequences when considering radiotherapy distortion requirements.