Human error is the main factor of gearbox assembly quality problem. Various aspects of uncertainty and not really knowledgeable which make human error cause complexity. This paper using Shannon entropy calculated complexity of operator's error, judging operation errors during the process. Firstly, it described the operation process of a gearbox factory assembly line operation from the engineering point of view. Second, according to Rasmussen and Reason two kinds of human error classification it classified operator's action error. And this paper gave weights to the operator's error based on expert evaluation and the outbreak of the difficulty. The human error accidents complexity measurement values provide foundation to the production quality control. The use of complexity to measure the quality accident of human error probability holds innovative and practical significance.