Compared with pure helium, the hydrogen and helium mixtures with proper proportion can improve the performance of pulse tube refrigerators in the temperature range from 20K to 40K. Because hydrogen is flammable and explosive, the safety of the system can not be ensured once the hydrogen and helium mixtures inside leaked out. It is necessary to study on the safety of hydrogen and helium mixtures. This paper presents the explosion limits of the helium and hydrogen mixtures with different proportion via the explosive test. The results are shown that helium can restrain the flammability of hydrogen well. When the mole fraction of hydrogen is less than 40%2 the explosive range reduces sharply. Especially, when the mole proportion of helium to hydrogen is greater than 10, the gas mixture doesn't explode no matter what is the proportion between hydrogen-helium mixture and air. But when the mole fraction of hydrogen is higher than 40%2 the hydrogen and helium mixtures still have a larger explosive range.