Mu method is a flutter solution technique based on frequency domain mu-analysis, which directly applies perturbation to dynamic pressure in aeroelastic equation of motion. In a previous study, the dynamic pressure perturbation was modeled as a purely real uncertainty, which could not guarantee the continuity of mu. To improve the continuity of mu, a small amount of complex perturbation is added to the dynamic pressure in addition to the real perturbation. Thus the mu method is improved with this mixed real/complex perturbation. Formulations and algorithm of the improved method are presented and validated with sample test cases. It is demonstrated that the improved mu method is more feasible and conservative than the original mu method due to the mixed perturbation.