Proof of work and proof of stake are commonly used in the current permissionless blockchains. These consensus protocols can be abstracted into a random process of selecting a node for accounting in a blockchain ledger. However, they are generally faced with resource consumption and vulnerability issues. We present PoC, a fair blockchain protocol based on proof of credit blockchain protocol. It is a special proof of stake protocol where the credit is a special kind of stake quantifying whether the nodes activity is beneficial to the system. Any nodes cannot change their credits arbitrarily. We demonstrate that our PoC protocol satisfies the security properties, including common prefix, chain quality and chain growth, under the assumption that the total credit the honest held is majority. Besides, we propose a self-audit mechanism and a hybrid incentive mechanism to enhance the security and stability. Finally, we explain that how the PoC protocol resist the double spending attacks and the selfish mining attacks.