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In the process of gear development, bench test is an important and necessary method to study and exam the dynamic performance of the developed gears. But, it is very difficult to do bench test when the developed gear size is too big or too small. Such as for the big and heavy gears, it is not convenient to be moved and measured. And it will take a long time and consume much more materials and energy if we use the full-size gear to do bench test. On the other hand, too small gear size also makes the installation and measurements are very difficult in the bench test process. Sometimes, because of the unsuitable gear size, it is difficult to do bench test with the existing test equipment. Model test method can help us to solve this problem. Model test method allow us to scale-up or scale-down the size of the gear to get the similar gear model according to the similarity principle, and use it to do bench test instead of the full-size gear, and then apply the test results and rules of the model gear on the full-size gear. In order to do similar gear model test, the similar principle between the model and original gears must be established. In this paper, the similarity principle of spur gear was analysed in the respects of geometry, mechanics and kinematics with dimensional method based on the similarity theory. It provides a theoretical basis for gear model test method.