Offset is making it crystal clear he wants to end his marriage to Cardi B -- and he wants to make sure they share custody of their kids.

According to legal docs obtained by TMZ Hip Hop, Offset is requesting joint legal custody of the 3 children he shares with Cardi ... although he does say Cardi's place should be the primary residence for the kids.

He's also asking the court to lay out each parent's child support obligations.

In addition, the ex-Migos rapper is asking the court to divvy up the couple's marital property, and thinks it's appropriate they each pay their own lawyer fees.

Cardi filed for divorce back in August and -- aside from the occasional holiday get-together -- has continued to live separately from Offset.