The thermal accumulative effect of liquids induced by laser illumination gives rise to the refractive-index change. On the basis of photo-acoustic equations, the mechanism on the transient-state and steady-state thermal indexes of the liquids can be analyzed, respectively. Moreover, the effects of the incident wavelength and focal length on the refractive index of deionized water are studied. Using surface plasmon resonance detection system that is highly sensitive to temperature alteration, an original liquid prism detection system is set up, and the resonance curve of the deionized water at various power levels is numerically simulated from which refractive-index change of 1.4×10-3 in the case of power change of 0.7 W is obtained. Then the experimental researches on thermo-optical effect of the deionized water are carried out by means of continuously operating laser of 980 nm wavelength with adjustable power levels, and the dependences of steady-state thermal index on different power levels are obtained. It is observed that the refractive-index change reach 3.35×10-3 when power change is 0.7 W. Finally, possible error sources between the experimental results and the theoretical simulation are discussed.