  We theoretically investigate the phase sensitivity with parity detection on
an SU(1,1) interferometer with a coherent state combined with a squeezed vacuum
state. This interferometer is formed with two parametric amplifiers for beam
splitting and recombination instead of beam splitters. We show that the
sensitivity of estimation phase approaches Heisenberg limit and give the
corresponding optimal condition. Moreover, we derive the quantum Cram\'er-Rao
bound of the SU(1,1) interferometer.
