Under room temperature, the influences of different parameters on the formation of femtosecond laser-induced aerosols with relatively large size in a small cloud chamber are studied. The experimental results show that, under sub-saturated condition, the aerosol number density increases with the increase of relative humidity, and 0.3-0.5 μm aerosols are dominated. However, when the environmental relative humidity is near to be saturated, the number density of relative large-size aerosols (with their diameter D≥0.7 μm) is enhanced mostly, and at the end, the number density of 1.0-2.0 μm aerosols is comparable with that of 0.3-0.5 μm aerosols. When the laser irradiation duration and tightened focusing condition are prolonged under this near saturated condition, the number density of different size aerosols increases simultaneously at the same degree, and ultimately the corresponding size distribution does not change too much. Theoretical analysis results show that environmental relative humidity plays a key role for femtosecond laser-induced relative large-size aerosol formation.