Flammable materials in the air will absorb the moisture in the air. To determine the effects of moisture, cotton fabrics were selected as experimental materials for our investigation. A series of experiments was conducted on conditioned cotton fabric samples with different widths (40, 60, 80, 100, 120 mm) and different moisture regain (0%, 2%, 4%, 6%, 8%, 10%, 12%, 14%, 16%, 18%). The surface temperature profile, flame height, and flame spread rate were observed and analyzed. It was found that with the increase of moisture regain, the pyrolysis zone of the sample became smaller monotonically. For the flame height and flame spread rate, the moisture brought a positive effect and a negative effect to them, resulting in a trend that keep stable first and then decrease. Moreover, the decrease rates of flame spread rate were greater for the wider samples, which were 0.81, 1.00, 1.27, 1.67, and 1.86 mm/s2, respectively for 40, 60, 80, 100, and 120 mm wide samples.