An industrial method for preparing a semi-aromatic copolyamide having a general formula A/10.T, wherein: A denotes a unit obtained by polycondensing a lactam or a C9-C12 amino acid; and 10.T denotes a unit obtained by polycondensing 1.10-decanediamine and terephtalic acid. The method includes a step for polycondensing the comonomers, the amino acid or the lactam, 1.10-decanediamine and terephtalic acid in the presence of at least one C1-C7 linear aliphatic acid. The level of foaming in a reactor is substantially reduced, during the polycondensation step, in relation to the level observed without C1-C7 linear aliphatic acid.