By use of a He-Ne laser beam to control the tip trajectory of a spiral, we quantitatively measure the relation between the spiral period and the intensity of the laser beam in an excitable ferroin-catalyzed Belousov-Zhabotinsky reaction. We find that the period dependence on light intensity has two different phases: a fast increase phase and a slow increase phase. Numerical simulations based on the modulated Oregonator model and the assumption that local concentration of ferriin decreases as laser power increases are conducted, which suggest two possible mechanisms leading to the increase of the spiral period with the intensity of the laser beam.