As important approaches of studying structural dynamic characteristics, modal analysis and modal test are widely used in the design and verification of centrifuge. Theoretical calculation and tests are carried out to acquire the low-order modal parameters of large geotechnical centrifuge TLJ-500. The results of modal frequency and vibration mode from the two approaches  are compared. It’s shown that the stiffness at the joints of the centrifuge plays an important part in determining the modal frequency, and the test model should be impacted in two different directions so as to get more modal parameters. Suggestions are put forward on the basis of the above analysis. The result of this paper is not only valuable for improving the precision of modal analysis on geotechnical centrifuge, but also for the subsequent design of centrifuge shaker.