The bidirectional erbium-doped fiber amplifier(Bi-EDFA) is a key component of the spatial domain active fiber cavity ring-down sensing system, and its gain characteristics and noise characteristics directly affect the stability of the sensing system. A low-gain and low-noise gain-clamped Bi-EDFA suitable for spatial active fiber cavity ring-down sensing systems is designed and optimized by Optisystem simulation software. The simulation results show that by optimizing the design, a Bi-EDFA with gain coefficient and noise coefficient of 15.35 dB and 3.52 dB respectively can be obtained, and the gain can be clamped.