This paper is aimed to study the fluctuation of the instantaneous availability (IA) for the parallel repairable system. First, the Markov model of IA for the two-unit parallel system with two states (up and down) under exponential distribution is established. Next, by Laplace transformation, the analytical solution of IA is obtained. The fluctuation of IA is analyzed according to the fluctuation theories. Then, in the same way, the fluctuation of IA for the n-unit parallel system with two states is analyzed, where the n units of the system are all same. Finally, some numerical examples are provided to show the effectiveness of the results in this paper.INDEX TERMS Fluctuation, instantaneous availability, Markov model, the parallel system.