An illuminating/sounding device activated by inflation for a balloon includes an illuminating lamp/sounder, batteries (4) and an outer shell (5). The outer shell (5) covers at least the illuminating lamp/sounder. The outer shell (5) has a gas entrance (51), a gas exit (52) and a gas passage (6) which connects the gas entrance (51) to the gas exit (52), for forming a running passage throughout the outer shell (5) for gas. The illuminating lamp/sounder has a sealing device through which said illuminating lamp/sounder seals up the gas passage (6). Before inflating, the illuminating lamp/sounder is idle; when the balloon is inflated, a pressure difference inside and outside the balloon is gained to force the illuminating lamp/sounder to move out of the gas passage (6) and then seal up the gas passage (6), herein a circuit of the illuminating lamp/sounder is connected and thus the illuminating lamp/sounder is activated to work.