The present work solves the dispersion equation for high viscous fluid surface waves. It is found that the relaxation characteristics of surface waves change from exponential decay to oscillation decay, and the dispersion equation for fluid surface waves makes a fundamental change when the waves step over the extreme point (Y ≈ 0.145) with the increasing of dimensionless number Y. To verify the correctness of the proposed result, the relaxation and viscosity-temperature characteristics of fluid di iso decyl phthalate (DIDP) with high viscosity fluid by using surface light scattering method while the temperature is in the range of 286.27-373.32 K. The results show that Y value decreases with the increase of the temperature. When Y<0.145, surface waves decay in exponential form and do not spread. The average deviation of measured viscosity and the viscosity in the reference is within ±1.5%. When Y>0.145, surface waves spread with oscillation decay form. The average deviation of measured viscosity and the viscosity in the reference is within ±2%. When Y→0.145, measured viscosity deviation increases sharply, and we cannot get reliable fluid viscosity. The present work will be helpful to understand the relaxation characteristics of high viscous fluid surface wave and offer the theoretical background for accurate and reliable measurement of the viscosity of high viscous fluid with surface light scattering method.