Abstract: YIG tuned oscillator combines linear broad band tunability and low noise characteristics. is used as VCO of PLL in the partial coverage type microwave synthesis experiment. More than ten thousand discrete frequencies from 2GHz to 4GHz, having the stability and acuracy of an ultra-stable quartz crystal oscillator, have been synthesized successfully. The influence of magnetic field hysteresis on the nonlinear performance of PLL is studied theo-retically. Three formulas of YIG phase locked loop with magnetic field hysteresis have been derived by analytical approximation method. They are the average frequency difference formula, the acquisition time formula and the acquisition range formula. The formulas have clear physical meanings. the experiment shows that the acquisition range formula agrees with the result of actual measurement. approximately.