Abstract: In order to further improve working performance of packaging mechanism of packaging cotton picker, rocker arm component as key parts of packaging mechanism of 4MZD-6 packaging cotton picker produced jointly was taken as research object, and lightweight design of rocker arm componentis were carried out by combining size optimization and topology optimization．Static and dynamic characteristics before and after optimization were compared and analyzed．Results showed that weight of rocker arm component was reduced by 9.46%, and total weight was reduced by 23.6 kg．After optimization, the maximum deformation of rocker arm component was basically unchanged, the maximum equivalent stress was reduced by 13.34 MPa, and natural frequency was increased．Static and dynamic characteristics of rocker arm component were further improved．Practical engineering application proved effectiveness and feasibility of lightweight design results．It could provide method guidance for lightweight design of other structures or other agricultural machinery of baling cotton picker．