Phase-locked loop(PLL) has broad application in the field of measurement and control, digital signal processing. The software PLL has became a trend of the development of PLL. With a great capacity of digital computing and signal analysis, A software and digital PLL is designed based on the basic principles and composition. Experimental and simulation results show that the software PLL has better performance of capture and tracking. The PLL designed by software has a higher flexibility and versatility than that by hardware, at the same time it has the advantages of simple in structure and flexible design parameters and etc.