Abstract: In this paper, nano SiO₂ microspheres with different particle sizes were prepared by the Stöber method, which were added to polymethyl methacrylate (PMMA)-based gel polymer electrolytes, and the size effect of nano SiO₂ microspheres and its effect on the performance of all solid-state electrochromic devices were studied. The results show that the volume ratio of ethyl orthosilicate and ethanol affects the particle size of the obtained nano SiO₂ microspheres. The larger the volume ratio, the smaller the particle size of the prepared nano SiO₂ microspheres. In the PMMA-based gel polymer electrolyte, when nano SiO₂ microspheres content is the same, the electrochemical window of the electrolyte first increases and then decreases with the decrease of nano SiO₂ microspheres particle size, and the ionic conductivity increases with the increase of its particle size. The electrochemical window reached its maximum at 150 nm of nano SiO₂ microspheres, where the increase in ionic conductivity became insignificant. Then, 150 nm SiO₂ is added to PMMA gel electrolyte with a content of 7wt%, which has the electrochemical window of 4.8 V and the conductivity of 1.13 mS/cm. Besides, the electrolyte was used to assembly transmissive all-solid-state electrochromic device with a structure: Electrically conductive glass (ITO)‖SiO₂/propylene carbonate (PC)-LiClO₄/PMMA‖metal-supramolecular polymer‖ITO. The obtained device can change color between light green and dark blue with a high contrast of 60.1%, and the device stability is significantly improved.