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The general principles of the unidirectional emission of a high-Q whispering gallery mode (WGM) in deformed circular microcavities are studied and presented in this paper. In the phase space of deformed microcavities, light in the chaotic sea leaks out from the cavity through specific refraction regions, the positions of which are determined by the stable islands and the unstable manifolds. By changing the cavity shape, we can tune the positions of the refraction regions, as well as fixed points, to achieve unidirectional emission with narrow angular divergence. The microcavity with high-Q and unidirectional emission makes high-efficient free-space excitation and collection possible; thus, it can be applied for far-field detection of nanoparticles and low-threshold lasers with collimated emission.