The existing settings coordination of time-limit for zero-sequence over-current relay of 500 kV autotransformer is discussed. The emphasis is laid on the coordination of zero-sequence definite time-limit over-current relays in middle-voltage（220 kV） winding with zero-sequence inverse time-limit over-current relays in high-voltage winding and common winding. Then two un-equations are derived according to the questions discussed. In the meantime the influence of dynamic changed zero-sequence fault current on the coordination， the check method for dynamic problem is analyzed. At last， an optional protection scheme of over-current relays in 500 kV autotransformer is discussed， and some problems to be solved are pointed out.