Multi-Tenancy (MT) is critical for Network Function Virtualization (NFV) platform as it reduces the cost of having network services by sharing expensive server resource among customers. This is especially critical for memory and CPU intensive services like Network Intrusion Prevention System (NIPS). In this work, we explore the issue of deploying a large-scale virtualized NIPS service on a commercial NFV platform. We observe that the scalability of NIPS service is not good when based on independent Virtual Machines (VMs). We propose a Multi-Tenant Aho-Corasick state machine data structure (MT-AC) and adapt it into NIPS to solve the issue. One MT-AC based NIPS service simultaneously checks traffic belonging to different tenants against a merged ruleset. The MT-AC data structure is very efficient as it eliminates the redundancies among tenants' signatures during the rulesets merging. Experimental results with real-world ruleset show that, in comparison with an independent VM-based solution, the MT-AC based NIPS service can support 2 to 4 times more tenants. Moreover, the throughput and latency performance of MT-AC based NIPS engine only degrades by 1%, when the tenant count increases from 8 to 128. The results validate that, the proposed MT-AC based NIPS service on NFV platform can support a large amount of tenants with a very low cost.