The behavior of the plasma induced by a laser on an alloy steel sample in air is observed by a time-resolved ICCD camera, and the plasma emission spectrum is simultaneously measured. The influences of the lens-to-sample distance (LTSD) on the plasma emission intensity, the plasma temperature and the plasma structure are investigated for a lens of 100-mm focal length. Its physical mechanism is studied, and the spatial distributions on the direction perpendicular to the sample surface are carried out for five different focal positions. It is demonstrated that the lens-to-sample distance has a considerable impact on the spectral signal, the plasma structure and the spatial distribution. The results show that the change of pixel intensity on the plasma image is consistent with that of the plasma temperature. The curves appear two peaks at the position of 92 mm and 107 mm respectively, and the values reach their maximum at 92 mm. It′s shown that the spatial distributions of the plasma temperature vary with LTSD on the direction perpendicular to the sample surface. Space evolutions of the different spectral lines in the plasma are also different.