To solve the problems in the construction of the internal quality assurance system in some colleges and universities, such as the difficulty in forming the quality culture, the imperfection of the system construction, and the insufficiency of the information platform construction, etc. By analyzing the five stages of the DMAIC model, the process of diagnosis and improvement is proposed, and from the definition stage, measurement stage, analysis stage, improvement stage, control stage, the improvement strategy of diagnosis, and improvement of internal quality assurance system is described. Through the application of the DMAIC model in actual cases, it can provide ide reference for the construction of internal quality assurance systems in colleges and universities. According to the BCC model in the DEA model, the author studies the relative effectiveness of the input and output of 12 university libraries in China. The results show that university libraries with effective technology accounted for 66.67%, while those without effective technology accounted for 33.33%, it also analyzes the investment changes of non-technical effective university libraries. This has an important reference and inspiration to improve the management of university libraries.