The invention relates to a 
battery energy storage 
system based power conversion 
system and a control method thereof. The power conversion 
system comprises a 
battery energy storage system, a bidirectional DC / DC (direct-current) converter, a grid-side 
current transformer, a filter circuit and an isolated 
voltage transformer. The 
battery energy storage system is in parallel connection with a direct-current 
bus of the grid-side 
current transformer through the bidirectional DC / 
DC converter; the grid-side 
current transformer, the filter circuit and the isolated 
voltage transformer are connected in sequence; and the isolated 
voltage transformer accesses to a 
power grid. The control method includes sequentially monitoring the 
battery energy storage system, the bidirectional DC / 
DC converter, the grid-side current transformer, the filter circuit and the isolated voltage transformer by a 
control circuit. Since the 
battery energy storage system utilizes battery cells connected in series and is in parallel connection with the common direct-current 
bus through the bidirectional DC / 
DC converter, circulation among battery packs is avoided, and capacity expansion of the system is facilitated. Further, by the aid of the monitoring control strategy for SOC (super 
capacitor state of charge) of the 
battery energy storage system and battery end voltage, excessive charge or 
discharge state of the 
battery energy storage system is avoided, service lives of the battery packs are prolonged, and comprehensive service cost is lowered.