Cloud providers have started to deploy various FPGA accelerators in their datacenters because the performance of many applications can be significantly improved by implementing their core routines in FPGAs. In conventional datacenters with FPGA accelerated servers, if a tenant wants to use FPGA accelerators, it requests for a VM instance residing in a server equipped with an FPGA accelerator. This paradigm to integrate FPGA into Cloud leads to poor resource sharing of the precious FPGA resources. In this paper, we propose FPGAPooling, an FPAG-enabled Cloud system where all FPGA accelerators are managed as a single resource pool and shared among all VMs. For a VM, instead of requesting the Cloud to run the VM on an FPGA accelerated server, at run time, when a VM needs to use FPGA acceleration, it requests an FPGA accelerator from the pool. After the VM finishes using the FPGA accelerator, it releases the FPGA accelerator back to the pool. We design a centralized scheduler to handle acceleration requests from VMs and assign each request to an idle FPGA accelerator at run time; We implemented a system prototype on IBM's OpenPower Cloud system. The key challenging of FPGAPooling is scheduling. We designed and implemented a group of scheduling algorithms for the FPGAPooling system. With extensive evaluations on both a small testbed and a large-scale simulation, we found that our algorithms can improve the average and tail job completion time by up to 7 and 4 times, respectively.