Hello ChengMY,
Thank you for using Apple Support Communities.
I understand from your post that your iPhone 6 (which is updated to iOS 9.3.5) cannot find a new Bluetooth device and keeps searching for it. What type of Bluetooth device are you wanting to pair with your iPhone? The reason I ask is that there are only certain Bluetooth profiles the iPhone supports. Here is a list of the Bluetooth profiles that are supported:
iOS: Supported Bluetooth profiles
If the device you are using utilizes one of the supported Bluetooth profiles, the steps in the following article may help resolve the issue for you:
Get helpconnectinga Bluetooth accessory with your iPhone, iPad, or iPod touch
Best Regards