Please follow the below steps to see if it helps.
1. Check your computer's hardware requirements: Make sure your computer meets the minimum hardware requirements to run Filmora. If your computer is not powerful enough, it may struggle to process high-quality video, resulting in low preview quality.
2. Optimize Preview Quality: Go to the Preview window and click on the "Settings" icon. In the "Preview Quality" drop-down menu, select "High Quality." This will increase the preview quality but may also slow down the preview playback.
3. Clear Cache: Filmora saves temporary files or cache files on your computer while working on a project. Over time, these files can accumulate and affect the preview quality. Clear your cache by going to "File" > "Preferences" > "Cache" > "Clean."
4. Update Graphics Driver: Ensure that your graphics driver is up to date. The graphics driver plays a vital role in the video rendering process, and outdated drivers can cause issues with the preview quality.
5. Reduce Preview Resolution: Lower the preview resolution to reduce the load on your computer's hardware. Go to the Preview window and click on the "Settings" icon. In the "Preview Resolution" drop-down menu, select a lower resolution.
6. Close Other Applications: Close any other applications running on your computer that may be taking up resources. This will free up resources for Filmora to run smoothly.