An illuminating balloon includes an inflatable envelope and a sectional bulb support integrated in the envelope. The sectional bulb support includes a horizontally freely turnable top support to avoid tangling of the bulb support with the envelope during inflating. The top support defines an upper locking space for connecting the whole bulb support to a radially inward projected ring member integrally provided at a top center of the envelope without causing water leakage thereat. The sectional bulb support is height-adjustable and adaptable to differently sized inflatable envelopes, and has an open bulb-protecting grid that does not block light sources to produce shadows on the envelope, and an air-relieving hole to prevent the envelope from fluttering while being quickly inflated.