Frequency domain interferometry (FDI) for medical acoustic vascular imaging was employed to detect multiple targets with high range resolution. The phase of each frequency component of an echo varies with the frequency, allowing target intervals to be estimated from the phase variance. When the interference within a range gate is coherent, the cross correlation between the desired signal and the coherent interference signal is nonzero. In this case the desired signal is canceled by a coherent interference signal. Therefore, we utilized frequency averaging to suppress the correlation of the coherent interference. The results of computational simulations using a pseudo echo signal showed that Capon method combined with a frequency averaging technique using a single reference wave presents a higher range resolution than that using a conventional method. In our experimental study, the range resolution of FDI combined with Capon method using a single reference wave was severely deteriorated as we approximated the autocorrelation function to the cross-correlation function of the echo and the reference signal. Ultimately, FDI combined with Capon method using multiple optimized reference waves gave a high range resolution than that using a single reference wave and could distinguish two target boundaries that were 0.05 mm apart.