In this paper, we obtain explicit branching laws for all unitary representations of Spin(N, 1) restricted to a parabolic subgroup P . The restriction turns out to be a finite direct sum of irreducible unitary representations of P . We also verify Duflo's conjecture for the branching law of tempered representations of Spin(N, 1) with respect to a minimal parabolic subgroup P . That is to show: in the framework of orbit method, the branching law of a tempered representation is determined by the behavior of the moment map from the corresponding coadjoint orbit. A few key tools used in this work include: Fourier transform, Knapp-Stein intertwining operators, Casselman-Wallach globalization, Zuckerman translation principle, du Cloux's result of smooth representations for semi-algebraic groups. Contents 1. Introduction 1 2. Preliminaries 4 3. Restriction to P of irreducible unitary representations of Spin(m + 1, 1) 14 4. Moment map for elliptic coadjoint orbits 27 5. Moment map for non-elliptic semisimple coadjoint orbits 34 6. Verification of Duflo's conjecture for Spin(m + 1, 1) 41 Appendix A. Unitary principal series representations 43 Appendix B. Representations of Spin(m + 1, 1) with trivial infinitesimal character 44 References 59 2010 Mathematics Subject Classification. 22E46. Key words and phrases. unitary representations, branching laws, discrete series, Fourier transform, moment map, method of coadjoint orbits.