In view of the contradiction between the fast charging demand and the cycle life of lithium-ion batteries, this paper took LFP/ fast-charging graphite lithium-ion battery as the research object, and proposed a stepped fast charging strategy according to its rate charging characteristics and phase transition characteristics. Combined with EIS and SEM, the cyclic behavior of flexible packaging lithium-ion battery was studied under specific single-rate direct fast charge and multi-stage constant current and constant voltage strategy. The experimental results showed that: 1) After 1200 cycles (charge-discharge voltage window: 2.50 V−3.65 V), the cyclic capacity retention rate of the stepped constant-current and constant-voltage fast charging mechanism lithium-ion battery was 91.46%, which was 2.28% higher than that of direct charging fast charging mode; 2) The resistance values of ohm impedance and charge transfer impedance were lower than those of direct charge; 3) SEM analysis showed that after the direct charge cycle, the appearance of graphite anode material appeared large cracks. The deterioration of the structure of anode material directly affected the continuous reconstruction of the SEI film at the negative extreme of Li+, which affected the capacity of lithium-ion battery and damaged the cycle life.