Abstract: Because of its advantages of high linearity, good symmetry and easy to implement, symmetric load is widely used when first put forward, especially in the VCO part of the charge pump PLL design.In this paper gave a detailed analysis of the symmetric load's IV characteristics based on approximate L1 model in theory, and the simulation results were fully consistent with the theoretical analysis by using SMIC 0.18μm CMOS process.In addition, this paper, from the point of affecting the performance of VCO, gives some guidance for the design parameters selection of the symmetry load.