With new technologies such as SDN(Software-Defined Networking) and NFV(Network Function Virtualization), network transformation is inevitable for network operators to make network programmable as well as to save cost. Consequently, applications can be deployed rapidly and automatically based on SDN and NFV. Correspondingly, security as one of key features for application also has to be provided dynamically and adaptively to meet different security requirements for today's rapid-increasing and evolving applications. However, traditional security appliances such as firewall are implemented as hardware-based middleboxes and placed on fixed locations in the network, so it is difficult to meet security requirements for today's applications. This paper proposes a new security service model based on security chain (i.e. security on demand) to provide customized security services dynamically and adaptively for today's applications. How to apply security on demand to the application (i.e. special vehicle speedup) is also elaborated.