The invention discloses a method for forecasting service life of a lithium ion battery based on capacity attenuation mechanism decomposition analysis. The method comprises the following steps: step one, establishing an equilibrium potential equation of a lithium ion battery to be tested by subtracting equilibrium potentials of positive and negative electrodes with combination of test data of the lithium ion battery to be tested, and then carrying out polarization modification to obtain the equilibrium potential equation; step two, establishing a multi-attenuation-mode decomposition model based on different attenuation mechanisms of the lithium ion battery, establishing a multi-attenuation-mode fitting equation of the lithium ion battery to be tested through a short-term aging test and forecasting attenuation trend; and step three, substituting a forecasting result of the multi-attenuation-mode fitting equation into the equilibrium potential equation of the lithium ion battery to be tested, and forecasting residual capacity. Through the service life forecasting method for carrying out decomposition analysis of different attenuation modes based on different capacity attenuations of the lithium ion battery, the problem that simple extrapolation is carried out according to the experiment test data or the attenuation of the service life of the lithium ion battery is simply ascribed to the single electrochemical mechanism is solved.