Theoretical ab initio calculations of the electronic structure were performed for the non-stoichiometric Mn1.75Co1.25Al Heusler alloy and compared with the electronic structure of the stoichiometric Mn2CoAl full Heusler alloy. Both compounds are assumed to have the L21-type crystal structure in the calculations, the non-stoichiometry is taken into account as a substitution of a Mn atom in a supercell. The calculation for the non-stoichiometric composition of Mn1.75Co1.25Al showed that taking non-stoichiometry into account leads to a decrease of the total magnetic moment. In comparison with the inverse type of Mn2CoAl, in both Mn2CoAl and Mn1.75Co1.25Al, the metallic type of the total density of states at the Fermi level was obtained in our calculations. In Mn1.75Co1.25Al, the total density of electronic states is found to be close to the one of the stoichiometric Mn2CoAl alloy in the majority spin projection, and in the minority spin projection spin polarization leads to the formation of the more intense peaks due to the appearance of an additional non-stoichiometric cobalt with a significant magnetic moment, as well as an increase in the magnetic moments of the other magnetic ions.