The invention discloses a folding telescopic gob-side entry retaining roadside packing flexible formwork and a construction method thereof. The flexible formwork is formed by connecting a flexible packing bag, a reinforcing net and positioning longitudinal bars through binding wires. The flexible packing bag is of a cuboid structure with an upper opening and a lower opening and is formed by sewing a whole fiber reinforced composite material. Only one connector is reserved in the whole flexible packing bag for sewing, and a lap joint region with the mechanical features the same as those of other regions is formed. The reinforcing net and the flexible packing bag are equal in height, and the width of the reinforcing net is half of the width of the flexible packing bag. The flexible formwork can be folded along the midline position of the square section where the positioning longitudinal bars are located and then transported underground, and during construction, the flexible formwork can stretch to the actual size. The method includes the steps that a top plate and a bottom plate at the construction position of a gob-side entry retaining roadside packing body are treated; single hydraulic props are erected to serve as temporary supports; positioning drill holes are marked and constructed; the flexible formwork is stretched and fixed; a packing material is injected. The flexible formwork is ingenious in design and convenient to construct.