Human centrifuge plays an important role in national aerospace development. However, deformation of the rotary arm and sound caused by deformation could be observed during the test. In order to guarantee the safety of equipment and test participants, it is necessary to evaluate the deformation of the rotary arm which is the largest structure in human centrifuge. In this paper, an estimation method for rotary arm deformation of human centrifuge based on modal method is proposed. Firstly, the basic principle of modal method is introduced. Afterwards, Forces on the arm while rotating is analyzed and D ‘Alembert's principle is applied, in order to transform rotation to a static process. A finite-element simulation of the arm with 2 different working conditions is carried out to verify the feasibility of this method. The simulation result shows that the proposed method could effectively estimate rotary arm deformation, and thus provides guidance for further practice.