In allusion to the deficiencies existing in current structural optimization algorithm of excavator boom such as the inefficiency in expressing and handling the constraints, the insufficiency in adopting the task knowledge to direct constraint handling, and the difficulty in obtaining and adopting the optimal process knowledge, a new method of the constraints expression and handling based on cultural algorithm for excavator boom structural optimization is put forward. The mechanism of hierarchical constraints expression and handing is established to improve the efficiency of the constraint handling in the optimal process with the cultural algorithm as frame and the task knowledge as guide. And the hierarchical topographical knowledge is formed to express tacit knowledge in the population space. Then the process of constraints expressing and handling as well as its interaction with the knowledge base is discussed under the direction of dual evolution in cultural algorithm while the aim of acquisition, expression and handling for tacit constraints knowledge in optimal process is realized. Finally, structural optimization of an excavator boom is taken as an example to demonstrate the effectiveness of this method.