The Internet of Things (IoT) confronts a complicated and changeful attack environment. It is necessary to evaluate the security risk of IoT dynamically to judge the situation of IoT. To resolve the above problem, a dynamical risk assessment method for IoT inspired by Artificial Immune System is proposed in this paper. The proposed method is made up of Detection Agent of Attack and Sub-system of Dynamical Risk Assessment. Furthermore, it adopts the technology of detector distribution. The simulation of immune principles and mechanisms in the real IoT environment is deduced by set theory in math. The attack detector evolves dynamically in the IoT immune environment. Its change forms the dynamical security risk value of IoT.