Hello and welcome to Apple Support Communities, Tian Yuan.
I see that you're having an issue when trying to get Siri to set a reminder from an email. I understand wanting this feature to work properly and I'd be happy to look into this with you.
Let's start by having you go through the steps in this article for help with Siri:
Get help with Siri on your iPhone, iPad, or iPod touch
Sometimes the wording that we use with Siri might be the problem too. The actual phrase you used should be okay, however try saying "Siri, set a reminder for this for tomorrow" while you have the actual email open that you want to be reminded about.
Let me know how this goes.
Thank you for using Apple Support Communities.
Have a great day!
