Polymeric nitrogen, as known to be synthesized at high pressure and high temperature, is considered as potential energy materials but impossible to be recoverable to ambient condition. Another alternative way is to search for nitrogen-rich materials and it thus remains a topic of intense interests. However, the lack of high nitrogen content compounds in previous work motived us to search for new nitrogen-rich material. Therefore, we performed a systemically structure prediction on Al-N system at high pressures using ab initio structure prediction method. Here, we found several novel nitrogen-rich materials in AlN system. AlN10 is found to be stable at 50 GPa and is dynamically stable at ambient condition, indicating it is a promising high-energy material if it is recoverable at ambient condition. Beside the newly predicted AlN10, we also found a AlN8 is stable at 100 GPa as well. This work highlights high pressure that is a powerful tool to design new functional materials.