The threshold of singly resonant KTP (θ=90°,φ=0°) optical parametric oscillator was reduced significantly (about 50%) by use of stimulated brillouin scattering (SBS) phase-conjugate Nd∶YAG laser as a pumping source comparing with the use of conventional multimode pumping source. Theoretical analysis shows that the OPO′s threshold energy fluence depends on the pulse width, non-symmetrical temporal profile (sharp leading edge), and beam mode structure. The compressed pulse-width, sharp leading edge and single spatial mode with good beam quality contribute to the OPO′s threshold reducing.