Based on the cross-gain modulation effect in Fabry-Perot laser diode (FP-LD), the 2.7GHz optical pulsed signal at 1552.30nm is converted to a continuous-wave light at 1548.22nm in experiment. The variation of wavelength conversion efficiency with injection optical power and wavelength are examined in detail. Our research results indicate that there always exists a fixed longitudinal mode for a given FP-LD under certain working conditions to maximize the extinction ratio of conversion signal and conversion efficiency when the longitudinal mode is locked by the probe light. Our theoretical analysis accords with the experiment results. Moreover, there always exist an optimum signal power and a detuning range between the probe light and the longitudinal mode to improve the conversion signal further under the same longitudinal mode locked by the probe light. The optimum longitudinal mode can be determined by the FP-LD characteristics and its working conditions.