A novel force sensor based on optical beam deflection was developed to investigate the mechanical effects during the pulsed laser ablation of a metal in liquids with different viscosities. The experimental results show that the targets in viscous liquids are affected by laser-generated ablation force and high- speed liquid-jet impulse induced by bubble collapse. In the same liquid, the amplitudes of the three kinds of forces increase with the increase of laser energy. With the increase in viscosity, the amplitudes of all three forces decrease, and the cavitation erosion is reduced. Larger viscosities make the bubble expand or contract more slowly, and bubble life-time becomes larger. Besides, the Rayleigh equation was revised, and the results are in good agreement with the experimental data.