The characteristics of martensitic transformation and magnetostructural coupling have been investigated for MnNiGe1-xGax (x=00.30) alloys. It has been found that the martensitic transformation temperature of the system can be deceased about 400 K by doping a low content of Ga to substitute for Ge. Adding Ga may weaken the hybridization and at the same time enhance the magnetic exchange interaction, which are responsible for the increase of the atomic moment and the Curie temperature in the austenitic phase. The structural and magnetic phase diagram was obtained to show some important points: 1) the decreased transition temperature successively pass through the Neel temperature and the Curie temperature; 2) the Curie temperature window still opens during the doping; 3) the magnetism can effectively affect the transformation behavior. The metamagnetization and the preparation influence on the transformation are also observed. These properties are significantly beneficial to the development of new magnetostructural materials.