Interrail is not linked to the EU. The fact that you have a non-EU passport is not enough information to be able to say whether you qualify for Eurail, Interrail, or maybe both. If your non-EU passport is a UK passport, then you only qualify for Interrail. Further, see article 5 of the booking conditions:

https://www.eurail.com/en/terms-conditions/booking-conditions

thank you for reply.

An Interrail Pass can be used by European citizens.

So, I have EU passport I can get Interrail Pass? or I live now in UK (non EU) so I need to buy Eurail? (but my home is in EU country, just because of university I am in UK now) My non-EU passport is not UK.  My journey starts from Spain. 

Don't mix up EU and Europe. EU passport or not is not relevant. Relevant is where you live. If you live in Europe (the UK is in Europe, also see the list of countries mentioned in the link above), then you qualify for Interrail. If you don't live in Europe, then you qualify for Eurail. If you live in Europe and also have a non-European passport, then you can also use Eurail.

Thank you so much.

One thing to consider, with your Interrail pass you can only use it in your home country (place of residence) on a max of 2 days.

If you do not plan to come to the UK then you should make sure it registers your place of residence as the UK, so you can travel in the other country in an unlimited way. Then you can happily travel with your UK passport on all trains in the rest of the Interrail network.

This is in no way affecting which passport you use for border crossings, only as evidence if ever asked by a train inspector in your EU passport country to show that you are a UK resident. (It is unlikely but may happen).
 

Thank you for your reply.

I do not have UK passport I just live in UK because of my university. My home home address is Bulgaria. I will start my rail journey from Spain bc my friends is there and I want to start with her. (Uk to Spain by plane and after that rail journey）

My rail plan: starts from Spain(bc of my friend) and then last destination is Bulgaria(home). 

So from this situation, my home is Bulgaria and I do not start journey from UK where I live now for just studies (and also UK resident consider as a part of EU from terms and conditions) so I can buy Interral pass?

You can buy an Interrail. The question is what country you want to indicate as your country of residence. Since you currently live in the UK (it doesn't matter that it's for university) and don't plan to travel in the UK, that sounds like a good choice, if you can prove you live in the UK, e.g. with a residency permit.

If you also have a non-European passport (you wrote you have 2 passports), then you can also buy a Eurail pass. The price is the same as Interrail but there is no restriction on travel in the country of residence.

5.2        The Customer is eligible to purchase a Eurail Rail Pass if they are a citizen or official legal resident of a country that is:

• not part of one the European Union Member states

and is not mentioned in this list:

• Albania, Andorra, Belarus, Bosnia Herzegovina, Faroe Islands, Gibraltar, Iceland, Isle of Man, Kosovo, Liechtenstein, Moldova, Monaco, Montenegro, North Macedonia, Norway, Russia, San Marino, Serbia, Switzerland, Turkey, Ukraine, United Kingdom or Vatican City.

5.3        Individuals who are a citizen or official legal resident of the countries defined in article 5.2 are not eligible to travel with a Eurail Rail Pass. Instead they  can purchase an Interrail Rail Pass through www.interrail.eu, the website of Interrail.

So from this, my resident of country is UK and UK is not eligible to travel with Eurail so I need to buy Interrail pass. and I need to bring two passport during journey. 

Thank you.

@Mia Popova 

Will the UK’s vote to leave the EU affect travel with an Interrail Pass?