We have studied the magnetic properties and magnetic-entropy changes of Mn_1.1Fe_0.9P_0.7As_0.3–xGe_x compounds with x=0, 0.05, 0.1, 0.15 and 0.3. X-ray diffraction (XRD) study shows all the compounds crystallize in the Fe₂P-type structure. Magnetic measurements show that the Curie temperature increases from 150K for Mn_1.1Fe_0.9P_0.7As_0.3 to 380K for Mn_1.1Fe_0.9P_0.7Ge_0.3. A field-induced first-order magnetic phase transition is observed above the Curie temperature for the compounds with x up to 0.15. There exists an optimal composition in which the first-order phase transition is the sharpest. The optimal composition for this system is x=0.1. The maximal magnetic-entropy change derived from the magnetization data is about 40J/(kg K) for a field change from 0 to 3T.