Interstitial La0.7Pr0.3Fe11.5Si1.5C0.2Hx hydrides with large magnetocaloric effect are prepared. It is found that the addition of carbon atoms can lower the rate of hydrogen absorption/release, which is beneficial to an accurate control of hydrogen content. The Curie temperature TC can be adjusted to the room temperature by changing the carbon/hydrogen content. The introduction of carbon atoms leads to a remarkable reduction of thermal/magnetic hysteresis because the first-order magnetic transition is weakened. The maximal magnetic entropy change of La0.7Pr0.3Fe11.5Si1.5C0.2H1.2 is found to be 22.1 J/kg K at TC = 321 K without hysteresis loss for a field change of 0-5 T.