I want to know that if a person is on house arrest threaten to do harm to another person is it a crime.

Would help to know which state or country it is in which this person is located?

Depends on the terms of house arrest, who they threatened and why. As well as local laws.

For example, if they were threatening a witness in their case it certainly would be a crime.

And unless (by contacting a victim or witness, for example) the person is violating the terms of the order allowing "house arrest" (as an alternative to jail), I don't see that "the terms of the house arrest", or indeed the fact that the person is under arrest, are relevant.

The real question is whether the person has committed a crime. That, in turn, would depend upon the criminal statutes of the jurisdiction and exactly what he or she is believed to have threatened to do.