The popularization and application of Cloud Computing have provided a new approach for users to get computing resources in recent years. Meanwhile, due to the advantages including programmability and power-efficiency, FPGAs have been applied to custom computing in many domains. Previous work has made resources of FPGA available under the cloud environment. However, the effective usage of FPGAs in the cloud requires efficient online task scheduling: to properly assign as many tasks from different tenants as possible to the FPGAs. In this paper, we propose a benefit-based scheduling metric to evaluate the task assignment Based on the metric, we accelerate task execution according to our benefit-based scheduling algorithms. By applying our benefit-based scheduling metric to a real OpenStack-based cloud environment, 60.32% computing resources are saved compared with the conventional throughput-based metric. Furthermore, a Replacement-Considering algorithm, which considers the task replacement, is proposed taking the characteristics of cloud into account The results show that our FPGA accelerated cloud system is 1.386 times faster than using the previous algorithm.