The time dynamics, threshold condition and energy output of a pulsed singlyresonant optical parametric ascillator(SRO) are studied. The theoretical and experimental results show that a SRO with a short wavelength wave resonant in the cavity(λsignal <λidler) isbetter, for improving the useful total output energy and energy conversion efficiency and reducing the threshold of the SRO,than that with a long wavelength wave resonant(λsignal >λidler).It also shows that injection of a seeder can reduce the threshold and increase the output energy of the SRO evidently.