Mixed-Mode frequency-modulated continuous wave (FMCW) synthesizer has proved to achieve the widest bandwidth and longest period of FMCW only using a 26MHz reference. In this paper, we propose a frequency-model for it both in time-domain and s-domain. Time-domain model proves why this architecture is suitable for FMCW application and offers designers enough details about this architecture in systemlevel. And s-domain model allows an accurate quantitative analysis and estimation of FMCW frequency error. The difference between the frequency error estimation based on our model and the measured results is less than 6.7%.