The lack of scalability is a leading issue of blockchain. By transferring transactions to off-chain, 2-party payment channels achieve instant transaction confirmation between channel users and enhance the blockchain throughput, thereby becoming a promising solution. By extending the channel from 2-party to multi-party, richer application scenarios could be supported. Meanwhile, new and exclusive requirements emerge in the multi-party off-chain payments, including robustness and flexibility. The robustness requires that the channel operation would not be impeded by any uncooperative channel member, and the flexibility guarantees that parties could join or exit the channel dynamically. However, all the current attempts either fail to achieve the new emerging properties or sacrifice some merits of 2-party channels. In this paper, we propose a new multi-party channel construction, Magma, which has good scalability. Magma outperforms the previous solutions to the multi-party payment channel for the following reasons. By canceling the heavy reliance on the cooperation of all channel members when implementing the channel operation, Magma achieves robustness. Magma also allows parties to join or exit one channel flexibly, without violating the balance security. Meanwhile, Magma's whole transaction process is performed off-chain, thereby inheriting the instant confirmation and low-cost features of 2-party channels. To guarantee the security of Magma, we formalize the multi-party channel's functionality and prove that Magma is secure in the UC framework. Moreover, our implementation and comparison show that Magma is practical and performs better than existing solutions in providing off-chain payment services.