The main factors which affect the precision of frequency transfer over optical fiber are analyzed,then a new scheme of dual- band frequency transfer over 25km optical fiber is designed and implemented bas ed on phase locked loop (PLL) compensation.Firstly,a reliability test scheme is designed on the condition of constant temperature to ascertain the instability of the fiber link,and all of the link transmission in dices are tested using a combination of electrical and optical test methods.Secondly,the link delay stability test scheme on the c ondition of changing temperature is designed,combined with the effects of the environment temperature and PLL parame ters on the frequency transfer,and an experimental platform for testing is built.Lastly,the transmission experiment over 25km optical fiber is carried out,and the changes of environment temperature and damping coefficient on the tr ansmission performance are also analyzed. The results indicate that the 10MHz high-precision atomic clock frequency sig nal which transfers over 25km optical fiber can still maintain a high stability with the effective compensation of PLL.T he scheme realizes stable link delay jitter within 0.5ns while the environment temperature varying slowly,whic h dramatically reduces the instability of transmission time delay in the fiber practical applications.