A passive Q-switched erbium-doped fiber laser based on a saturable absorber of single-walled carbon nanotubes coupled with a Sagnac loop is proposed. The output laser properties of the laser was experimentally investigated. Single-walled carbon nanotube saturable absorbers were created by light deposition with a 75% transmittance. A Q-switched laser was built to modify the resonators Q value based on the single-walled carbon nanotubes saturable absorption properties. When the Sagnac loop filter was placed inside the optical fiber ring cavity, the Q-switched pulses could achieve fine filtering thanks to the filtering effect produced by the Sagnac loop structure. The Q-switched lasers operating threshold is 800 mW. When the pump power is 830 mW, the laser can produce a stable 1 530.4 nm laser output with an output power of 12.3 mW. The repetition frequency is 210.7 kHz, the corresponding pulse period is 4.76 μs, the pulse width is 2.19 μs, and the maximum pulse energy is 58.37 nJ .