Generally speaking, the Eastern Han Dynasty and the Western Han Dynasty together count as the Han Dynasty.

In this way, the Han Dynasty was the longest dynasty from 206 to 220 years ago, a period of 425 years.

If calculated separately, it is the Tang Dynasty (6 18-904), followed by the Ming Dynasty (1368- 1644).

If you count the dynasties before the Qin dynasty (generally not so)

The longest is the Zhou Dynasty, which existed for about 800 years.