A video refutes the self-defense claims of a Queens teen charged with fatally stabbing a man on Christmas Eve, authorities said yesterday.

Cyan Brown’s bail was set at $300,000 after she pleaded not guilty to a manslaughter charge yesterday in Queens Criminal Court.

Brown, 16, and Thomas Winston, 29, got into an argument at a fried-chicken restaurant in Long Island City.

After exchanging words, Winston left the restaurant, and Brown came outside after him with a knife, police said.

Winston slipped and fell on a patch of snow. Restaurant security video shows that while he was on the ground, Brown jumped on him and stabbed him, said Queens DA Richard Brown.

Winston died of a single stab to his heart, authorities say.

Prosecutors see no way the teen can claim self-defense.