The invention discloses an Li traction battery series parallel change-over charging method and a charging device. The charging device comprises a power main loop comprising a rectifier circuit, a DC filtering capacitor, a pre-charging circuit, a voltage sensor, a switch bridge circuit, a charging current sensor and a series parallel change-over switch, a control circuit board comprising a parallel branch signal unit, a switch signal drive control unit, a coil signal circuit, a voltage current signal circuit, a control processor and a signal display unit, a serial charging circuit comprising a filter inductor, a filter capacitor and an output voltage sensor, and a parallel charging circuit comprising a primary coil, a feedback coil, an iron core, a plurality of secondary coils of a multi-winding transformer and a plurality of series parallel charging unit. The pre-charging, serial charging and parallel charging are carried out by the charging method according to a set pre-charging voltage value and an end voltage value. The method and the device provided by the invention reduce the cost and volume of charging devices, maintain the balance of charging capacity of every Li traction battery electrical core, ensure adequate protection and best working status for each battery unit and improve safety and service life cycle of Li traction battery.