Abstract: When the age of a Cf source exceeds 20 years, the relative contribution of Cf-250 and Cm-248 isotopes will increase gradually. Then the total neutron emission rate will not follow the simple index law determined by Cf-252. Based on the laws of radioactive decay and spontaneous fission for every isotope in Cf source, we put forward a method of determining isotope components (Cf-252 and Cf-250) of the sealed Cf source to rapidly estimate the total neutron emission rate of a Cf source over 20 years. The neutron emission of a Cf source about 30 years old was measured relatively by a long counter at one point. Then isotope components of the sealed Cf source were determined. The numbers of Cf-252 nuclei and Cf-250 nuclei at the beginning time of the source were 4.4521016 and 7.7461015. The relations between the total neutron emission rate and the age of the Cf source is calculated based on its components. The calculated results are in accordance with the measured results, and the relative difference between calculation and experiment results is less than 0.3%. Furthermore, the relative contribution of each isotopes, including Cf-252, Cf-250 and Cm-248, to the total neutron emission rate is analyzed. The result shows that the relative contribution of Cf-252 is reduced to 67.65%, while those of Cf-250 and Cm-248 are increased to 29.32% and 2.94% respectively when the age of the Cf source has reached 30 years.