A laser field that contains both additive and multiplicative noise are analysed based on a full saturation laser model. The numerical solution of intensity correlation time T c as well as the exact formulas of effective eigenvalue λ eff are calculated. Compared with the experimental measurements, excellent agreement is obtained for the full saturation laser theory, while relatively large errors occured in the third order laser theory and the measured data. By best fitting the experimental data of both Tc and λeff with the full saturation laser model, the cavity decay rate K of the laser sytstem can be determined, therefore it provides a theoretical evidence for measuring K.