We construncted the mathmatic formular of the linewidth in optical parametric generation and amplifier caused by the sponetaneous parametric emission using quantum theory, and studied, computed and compared the influnce to the linewidth of other physics factors. It is shown that the linewith of parametric light is very narrow far away from the degeneracy point, and the linewidth begins to enlarge quickly at the degeneracy point; And the divergence angle of pumping beam and the off axis pumping influenced the linewidth more seriously than other factors do. These results are important for narrowing the linewidth of parameteric light.