We demonstrate the data transmission characteristics of a 1.96 µm laser in an indoor simulated smoke channel. An active mode-locked thulium-doped fiber laser at 1961.63 nm can be generated, and the repetition rate is 2.11 GHz corresponding pulse duration of 12.47 ps. Therefore, the pulse can be modulated by a 2.11 Gb/s digital sequence to act as a carrier light source. The transmission characteristics of signal light under different visibility conditions of 0.5, 0.05, and 0.005 km are studied. Compared with the back-to-back conditions, the signal-to-noise ratios of the carrier can decrease after passing through the smoke channels, which are 3.93, 7.1, and 9.08 dB, respectively. At the same time, the received power jitter can increase from ±0.16 to ±1.14dB. The sensitivity can be -19.52dBm at the visibility of 0.005 km. The experimental results show that thulium-doped mode-locked laser has an excellent performance in a smoke channel.