Quasidiffusions are, by definition, time-changed Brownian motions on certain closed subset of $\mathbb{R}$. They admit an explicit representation of Dirichlet forms in terms of so-called speed measures. The Fukushima subspace of a Dirichlet form means another regular Dichichlet form on the same state space but having a smaller Dirichlet space. In this paper we aim to solve the problem of Fukushima subspaces for quasidiffusions. The main result obtains all Fukushima subspaces and characterizes their structures. In addition, we will also give criteria for the uniqueness of Fukushima subspaces and the existence of minimal Fukushima subspace.