The 42-year-old was challenged by police just before 9pm yesterday but failed to respond so the electronic weapon was used.
The man had been shouting in the road and appeared to have a knife that he was pointing at his throat.
He suffered a minor head injury when he fell and was taken to hospital as a precaution. He was later arrested on suspicion of possession of a knife and affray, and remains in custody.
Two police officers from the Diplomatic Protection Group fired the Taser.
It follows a separate incident last month when police outside Buckingham Palace used a Taser on another man who was also accused of brandishing a knife.
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"I think there are a number of things I didn't do correctly in this case", said Ademola Dada.
Mr Johnson vowed to lie down in front of the bulldozers when he was elected MP for the west London constituency of Uxbridge & South Ruislip
The course is designed to help spot the signs of suspicious behaviour.