The invention provides a light-gas gun downhole explosion shell and a using method thereof, and belongs to the technical field of explosion. The light-gas gun downhole explosion shell comprises a first semicircular-cylinder-shaped shell body and a second semicircular-cylinder-shaped shell body. The first shell body and the second shell body are provided with connecting buckles correspondingly. The first shell body and the second shell body are buckled and form a cylinder-shaped structure with the two ends being closed through connecting and fixing through the buckles. A sealing strip is arranged at the joint of the first shell body and the second shell body. A lead hole and a gas guide tube holes are further formed in the joint of the upper end of the first shell body and the upper end of the second shell body. The problems that water monitor holes are complex to operate and strict in storage space are solved, and lots of dust and toxic gases generated after explosion of explosives are greatly reduced.