The magnetostructural coupling in off-stoichiometric MnCoGe alloys is realized within a wide Curie temperature window (up to 96 K) through tuning the Mn/Co ratio and it results in a significant entropy change (up to − 30.8 J K−1 kg−1). Two scenarios have been studied, i.e. the Co- and Mn-rich compositions, respectively. In both scenarios, the orthorhombic-hexagonal structural transition temperature can be shifted into the temperature range between the Curie temperatures of the orthorhombic and hexagonal phases, enabling the magnetostructural coupling between two phases. Although the magnetostructural coupling can be achieved in both scenarios, the Co-rich alloys show a much higher entropy change and a wider Curie-temperature window than the Mn-rich alloys. Based on the systematic structural and magnetic analyses, the composition-dependent phase diagrams for both scenarios have been constructed.