The flatness of a certain batch of aluminum alloy thin-walled laminations is the key quality control object. The calculation result of the process performance index Pp U in the initial manufacturing stage showed that the process capability was insufficient. In order to solve the above problem, through analysing the structural characteristics of the lamination and combing the production process, an end factors’ analysis model of the out-of-tolerance flatness was established; by means of investigation and experimental verification, the main causes of out-of-tolerance flatness were confirmed. Targeted improvement measures such as optimization of mould inspection strategies and replacement of qualified moulds had been taken. After the improvement, using the MINITAB software, the Xbar-R control chart and process capability chart were drawn, and the calculation and analysis of process performance index Pp U were completed. The results showed that the improved production process was in a state of statistical control and the process capability was sufficient. The effectiveness of the improved measures was verified.