TbMnO3 is an important multiferroic material with strong coupling between magnetic and ferroelectric orderings. Incommensurate magnetic ordering is suggested to be vital for this coupling in TbMnO3, which can be modified by doping at the site of Tb and/or Mn. Our study shows that a self-doped solid solution Tb1−xMnyMnO3 (y≤x) can be formed with Mn doped into the site of Tb of TbMnO3. When y is small Tb1−xMnyMnO3 shows both ferroelectric and incommensurate magnetic orders at low temperature, which is similar to TbMnO3. However, if y is large enough, a commensurate antiferromagnetic ordering appears along with the incommensurate magnetic ordering to prevent the appearance of multiferroicity in Tb1−xMnyMnO3. That is to say, the magnetoeletric coupling can be broken by the co-existence of a commensurate antiferromagnetic ordering. This finding may be useful to the study of TbMnO3.