Question on the decidable of Turing Machine

I am a bit confused about using the subset of the turning machine to prove the desirability of the turning machine.

If I have a Turing Machine M and we have already know M has a single halting state. If we already the machine takes a string was the input and reaches to the state q0, is it possible to prove that M is decidable by considering constructing a new Turing Machine that halts on q0?

Thanks a lot!