A diode-pumped single-path mid-IR fiber gas laser is reported. 3.1~3.2 μm laser emission is generated in a 2.3 m low-loss negative curvature hollow core photonic crystal fiber (HC-PCF) filled with low pressure acetylene, pumped by a modulated, amplified, tunable 1.5 μm diode laser. The maximum energy conversion efficiency is higher than 14.5% at 200 Pa, and the minimum laser threshold is lower than 100 nJ at 100 Pa. This research potentially provides an effective rout to obtain compact, high-power mid-IR fiber gas laser.