Where did you make the reservations? It is only they who can make the refund. You should be able to get a refund on the Eurostar if it was a connecting journey. As I said you will need to apply through the company that made the reservations for you.

I use Eurail pass and make the seat reservation at Eu rail website.  What should I do?

The email which had your reservation pdf tickets will have a contact detail for you to apply for a refund. You’ll need to be patient as there are thousands of people affected by the Thalys cancellations.

The ticket shows that exchangeable before departure by paying £15/20€ fee per leg per person. Non refundable. So is it cannot refund? I need cancel my trip how to do

Only the issuing office can do this. As the train was cancelled you are due a full refund on the Thalys leg  and also the Eurostar if it was a recognised connection

So Am I need to write an mail to Eurail explain my case ?

Yes

But this is a rather strange route: Tls runs on the same tracks-hi-speed, from Paris to the north as far as LIlle-so you double back some 222 Kms. Tls is plagues with many sets out of service due to repairs and no parts.

What is the trip you really intend to do? We see so many rather awkward routings by people who have no idea what they do and think they can plan travel-agent. IF you want to come back to UK you had beter rearrange so you can still get on that €*-if not you then are in deep trouble