A pulsed optical parametric oscillator (OPO) was demonstrated for the purpose of wavelength-tunable yellow output with narrow spectral line width. The OPO pumped by the green output of an acousto-optic Q-switched Nd:YAG used a type II phase-matched KTiOPO₄ (KTP) crystal as the nonlinear gain medium and a folded cavity arrangement. The OPO was designed to have the idler wave tunable in near infrared oscillated in the cavity, which was further frequency doubled to generate the wavelength-tunable yellow output by using a LiB₃O₅ (LBO) crystal with type I non-critical phase matching scheme. A fused silica etalon was inserted in the idler wave cavity to narrow the idler wave and the resultant yellow spectral line width. The wavelength of the yellow output obtained could be tuned over 587.2-595.2 nm, within which the maximum average output power of 2.89 W was obtained at 591.2 nm, under an incident average green pump power of 24.0 W. The beam quality factor M² was 3.4. The conversion efficiency from the green pump to the yellow output was 12.0%. The pulse width at the maximum output power was 37 ns, and the peak power was 7.8 kW. The spectral line width of the yellow output was 0.15 nm, which was narrowed effectively compared with that without etalon in the OPO cavity.