Lithium triborate (LBO) possesses desirable nonlinear optical characters, as well as stable chemical and mechanical properties. Its dispersion is sensitive to the change of the crystal temperature, which allows this nonlinear optical crystal to be used to achieve noncritical phase matching. This paper demonstrates a femtosecond synchronous green-pumped singly resonant optical parametric oscillator (OPO) based on lithium triborate. As the pump source, a frequency-doubled mode-locked Yb-fiber laser-amplifier system is used, which can produce femtosecond laser with high average power and large mode field area. By adjusting crystal temperature, tunable high power femtosecond laser can be obtained from red to near infrared using noncritical phase matching method. The signal pulses of the optical parametric oscillator have a wide spectral tuning range of 670~880 nm and idler pulses are tunable in the range of 2320~1270 nm. At a pump power of 3.4 W, the output highest average power of 660 mW at 694 nm central wavelength is achieved with the pulse duration of 132 fs and the slope efficiency of 19.4%.