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In the rotating wave and slowly varying envelope approximations,the density-matrix motion equations for the three-level cascade system equation are solved under the influence of driving-field phase diffusion.The exact linear analytical solutions of the three-level cascade system with the driving field having the phase fluctuation in the steady state are obtained.The effects of phase diffusion on absorption,dispersion and population difference in a three-level upper-ladder system and on the time of steady-state limit in the course of time evolution have been analyzed by using the numerical simulation from the steady linear,analytical solution and density-matrix motion equations.Gain of the probe field will be decreased due to the driving-field phase diffusion.The presence of the finite linewidth induced by the driving-field phase diffusion prevents the ladder system from obtaining a high refractive index along with zero absorption.Variation of the linewidth can not change the property of the lasing in the upper-ladder system.The linewidth as a result of the driving-field phase diffusion has influence to the time of steady state limit and the phase diffusion leads to a decay of the coherence in the course of time evolution.