Crowdsourcing refers to a distributed problem-solving mechanism that solves complex problems which are difficult for a single individual to solve by integrating the unknown free and voluntary masses on the Internet. In the existing crowdsourcing systems, requesters and workers are usually required to use the crowdsourcing platform as the trust center, and the payment of the fee depends on the third-party central payment institutions. This centralized service mechanism has a large security risk. Once these centers are attacked, or the centers are doing evil, it will bring greater losses to the crowdsourcing parties. Based on the blockchain technology, we propose a new and decentralized trusted service mechanism for crowdsourcing system. The crowdsourcing service process is divided into nine stages, and the smart contract controls the execution of each step in each stage. In addition, the payment is completed by transferring within the blockchain. Finally, we develop smart contracts to conduct experiments based on Ethereum, and conduct comparison experiments. The experimental results show that the effectiveness and applicability of the service of the crowdsourcing system service mechanism without the central institutions.