A novel nonlinear PLL which can be used in VCO nonlinearity compensation was presented. By using the SAW(Surface Acoustic Wave) delay line to delay the FMCW signal, the IF signal can be locked on a reference frequency after mixing with the delayed VCO signal frequency and the undelayed. Based on this idea, the loop phase model of this PLL was analyzed and simulated by using Agilent ADS. The CAD simulation results show that this kind of PLL can be used in VCO(Voltage Controlled Oscillator) nonlinearity compensation circuit conveniently.