Having the correct resolution of phase ambiguity is critical in achieving high-accuracy ranging for frequency-modulated continuous-wave (FMCW) radar. An incorrect resolution may result in cycle slips in ranging results. This article investigates the causes of cycle slips and quantifies their probability of occurrence. We propose a dual-frequency (DF) phase ambiguity resolution method for achieving high-accuracy ranging in FMCW radar. The method combines two FMCW radar signals of different frequencies into a virtual signal, increasing the wavelength and enabling accurate phase ambiguity resolution while minimizing the probability of cycle slips occurrence. Simulation tests demonstrate the superiority of our proposed method over conventional approaches. To validate our approach, we set up a DF platform that includes two FMCW radar modules operating at different frequencies (60 and 77 GHz). The experimental results confirm that our proposed method significantly improves phase ambiguity resolution performance.