A nanosecond Er/Yb co-doped all-fiber dual-cavity laser with fiber-based passive Q-switch is reported. The laser oscillator is constructed in a linear dual-cavity, and can achieve the efficient, stable nanosecond operation based on the saturable absorber of the single- mode double- cladding Er/Yb co- doped fiber and the crossmodulation of the dual-cavity. The maximum output power is 2.2 W, and the minimum pulse duration is 173 ns with the central wavelength of 1570 nm. Meanwhile, sequential nanosecond pulses can be obtained at the repetition rate of 14~156 kHz.