The linewidth of KBe<sub>2</sub>BO<sub>3</sub>F<sub>2</sub> crystal optical parametric oscillator (KBBF-OPO) has investigated using theoretical analysis and simulation in type-I phase matching (PM). The Linwidth ranges of 81~250nm with the spontaneous
parametric emission, 230~690nm with the divergence angle of pumping beam, and 19~96nm the linewidth of signal light
are obtained, respectively. It is shown that the linewidth of signal is very narrow far away from the degeneracy point and
the linewidth increased near the degeneracy point and given the results that KBBF-OPO has smaller linewidths than that
of CLBO-OPO in type I. The divergence angle of pumping beam and the off-axis of pumping beam influenced the signal
linewidth more seriously than other factors do. These results are important for compression the linewidth of OPO signal
and useful for construction high power KBBF-OPO laser.