The solution of a Rayleigh-wave inverse problem may potentially deviate from the realistic shear-wave velocity structure due to nonuniqueness. To overcome such deviation, it is necessary to understand the source of nonuniqueness and situations that may give rise to the nonuniqueness. In this study, the existence and formation of the nonunique solutions in an inverse problem are demonstrated by modeling the solution space of a synthetic surface-wave inverse problem and investigating the major causes that might engender nonuniqueness, namely (1) the inversion convergence threshold, (2) ambient noise, (3) corner frequency of the recordings, and (4) the water level. Regarding the severity of nonuniqueness in the phase-velocity inverse problems, a technique is proposed to improve the inversion that exploits the match between the synthetic and observed time series used as a posteriori information for constraining the realistic velocity structure. Through a synthetic example, the effectiveness of such method is tested and demonstrated effective.