What's the difference between Mandarin in Qing Dynasty and Mandarin now?

There is little difference in pronunciation. Today's definition of Putonghua as "Mandarin" is based on the pronunciation of the late Qing Dynasty and the early Republic of China. For different pronunciations of individual words, please refer to Taiwan Province Putonghua.

In terms of written language, it is also very close. You can easily understand the novels of Dream of Red Mansions in the middle of Qing Dynasty, not to mention the fact that the late Qing Dynasty lasted until the Republic of China. Needless to say, spoken language is basically vernacular, and classical Chinese is limited to literati, but literati are a minority after all and are only used when publishing official articles. People who usually speak classical Chinese can't understand it. The classical Chinese in Qing Dynasty is easy to understand, and it is not as difficult as before Qin Dynasty.

So, if you travel through the Qing Dynasty, you are alive, you can understand and write words, but if you travel through the Ming Dynasty, you basically talk to ducks.