The incident happened on Parliament Hill on Wednesday morning while the red-coated guards were performing the daily ritual.
Paramedics say the 20-year-old male was treated for a severe stab wound to his forearm caused by his own rifle-mounted bayonet.
Canadian Forces medics provided first aid until paramedics arrived.
The man was treated for severe blood loss and remains in serious but stable condition in hospital.
The changing of the guard is a tourist attraction performed daily on the lawn in front of the Centre Block in the summer months.
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