The difficulty of external audit of mechanical science and technology is not invariable, which mainly depends on many factors such as complexity, innovation and scope of application, because different products or schemes have different technical difficulties and requirements in design, manufacture, installation and use. Of course, for those relatively simple products or schemes, external audit is relatively easy, and with the improvement of the complexity and innovation of products or schemes, the difficulty of external audit will also increase. It should be noted that in the process of external audit, we should fully understand and master the audited products or programs as far as possible, and accurately grasp their technical difficulties and requirements, so as to better complete the external audit work.