Project saves even I chose not to.

Hi, I’m just new in Unity3D, And I noticed that whenever I exit unity and not save,after I reload my file,the last thing I did is still in there. It’s really inconvenient because everytime I made mistakes that cannot be undone,I must Reload the backup of my package again because unplanned changes are always saved. Does unity really behave like this? or is this a bug?

It’s not a bug; all project changes are saved when you quit. It would be annoying if it didn’t work like that, because then you’d be forced to remember to save the project and the scene, and project settings are the sort of things you’d expect to be persistent without having to always manually save them. Changes to scenes are only saved if you explicitly tell Unity to save.