Why did the emperor protect Qin Huan's secret?

This is because once Qin Huan pretends to be the emperor, it has a wide influence, so he can't stay out of it.

In "Aquilaria Resinatum", Qin Huan abandoned the inner alchemy of the Shura clan in order to go undercover, and was washed by the Book of Changes, paying an unimaginable price, and finally gave birth to a fairy and became an emperor, but his efforts turned into a joke. All the undercover agents of Shula tribe have become abandoned children of Qin Huan. Qin Huan was unwilling, so he took refuge in Zundi and slaughtered the Shura clan. After Emperor Zun knew that Qin Huan was a fake Emperor Zun, he used Gankun to woo the old staff of the Shura clan and prepared to establish his own power center.