The invention discloses an automatic cap opening and closing device. The automatic cap opening and closing device is composed of a roller assembly, a cap opening and closing assembly, a cap opening and closing executing mechanism and roller brackets, wherein the roller brackets are fixed on a foundation surface and are located on the two sides of a roller; the roller brackets are used for supporting the roller and relative parts thereof; the roller assembly is used for mixing materials through overturning the roller; the cap opening and closing assembly is used for realizing the sealing of the top of the roller so as to prevent the materials from scattering in an overturning process of the roller; and the cap opening and closing executing mechanism can realize automatic cap opening and closing through the retraction of an air cylinder and the rotation of a pneumatic motor. The automatic cap opening and closing device adopts a roller rotary mixing manner so that the raw materials can be relatively uniformly mixed; the device is pneumatically driven so that the speed is rapid; and the production safety in an operation process of processing and blending combustible and explosive powder is improved. The automatic cap opening and closing device has a high automation degree in the process of blending the combustible and explosive powder; and field unmanned operation is realized, the labor intensity is alleviated and the production efficiency is effectively improved.