thats strange usa visit refusal can also impact on canada study permit? if this is the case than I think my wife won't get canadian study permit please tell me should I let my wife apply ? what are the chances?
Of course regardless of where you get refusal it will affect your future applications anywhere. You will find it difficult to go to Australia, New-Zealand, Canada and most western European countries with multiple refusals.
If you are able to explain why you or your wife got the refusal and what has changed since then, then go ahead apply.