Aiming at cascading overload phenomenon in power network and considering structural integrity, the fluctuation phenomenon is researched into. Firstly, according to the thoughts that a power network is often weakened by cascading failure of branches, a statistical fluctuation model which can represent the structural integrity of a power network is proposed. Then an example in IEEE 39-bus system with continuously increasing of the injection power of nodes is given. The analysis results show that the fluctuation value of the relative parameter increases with the increase of the total load of the power system, and a critical point exists in that course. At last, in order to further prove that regulation, another example with IEEE 14-bus is analyzed.