A nanosecond Tm-doped all-fiber dual-cavity laser with fiber-based saturable absorber is reported. The laser oscillator is constructed in a linear dual-cavity. The system uses a 1550 nm continuous-wave fiber laser as the pump source and fiber Bragg gratings as the wavelength-selective devices. Based on the saturable absorption characteristics of the Tm-doped fiber and the unique design of dual-cavity, the stable nanosecond operation is achieved. Meanwhile, the effect of the single-cladding Ho-Tm co-doped fiber and double-cladding Tm-doped fiber as the gain media on the laser output performance is studied. The maximum output power of the system is 256.3 mW, and the minimum pulse duration is 87 ns with the central wavelength of 1993 nm. The sequential nanosecond pulses can be obtained at the repetition rate of 20.0-33.3 kHz.