High performance computing (HPC) applications are becoming more data-intensive and produce increasingly large I/O demands on storage systems. New storage devices such as SSD which has nearly no seek latency and high throughput have been widely used together with HDD to serve as a hybrid storage system. To solve the I/O bottleneck problem, existing hybrid storage solutions such as Burst Buffer have been proposed as intermediate layer between clients and disks to absorb burst I/O requests and improve write performance. However Burst Buffer needs sufficient SSD space to meet the maximum burst I/O requests which is still a costly solution. In this paper, we propose a hybrid architecture called SSDUP (an SSD write buffer Using Pipeline) for HPC storage systems, which uses NAND flash based SSD as a write-back buffer for HDD. With our efforts, SSDUP can achieve a good performance by using limited SSD space.