The difficulty of distributed system security and reliability testing lies in the lack of dynamic assessment in the system brittleness. Therefore, a new concept called brittle relative entropy is proposed to measure the system brittleness, and assessment methods are given. The brittle relative entropy can dynamically measure the relative distance between the current probability distribution and the collapse distribution, which can effectively assess the current state of the system and analyse the probabilistic risk of brittleness. The simulation results show that the brittle relative entropy can be used to measure the brittle characteristics of the system. When the brittle relative entropy is closer to the brittle distribution system, the brittle relative entropy is smaller.