The invention discloses a straight-flow steam generator which comprises a shell and a plurality of heat exchange tubes. The shell is sequentially divided into an oil inlet chamber, an oil outlet chamber, a preheating chamber, an evaporating chamber, a dehumidifying chamber and an overheating chamber from bottom to top, a heat transfer oil inlet is arranged on the oil inlet chamber, an inner tube plate is arranged between the oil inlet chamber and the oil outlet chamber, a heat transfer oil outlet is arranged on the oil outlet chamber, an outer tube plate is arranged between the oil outlet chamber and the preheating chamber, a feed-water inlet is arranged on the preheating chamber, a steam outlet is arranged on the overheating chamber, each heat exchange tube comprises an inner tube and an outer tube, the bottom end of each inner tube extends into the oil inlet chamber and is fixed onto the inner tube plate, the top end of each inner tube extends to the upper portion of the corresponding outer tube, the bottom end of each outer tube extends to the oil outlet chamber and is fixed onto the outer tube plate, the top end of each outer tube is provided with a closed structure and extends to the overheating chamber, and each inner tube and the corresponding outer tube form an interlayer structure. The straight-flow steam generator effectively solves the problem of poor heat expansion, and has the advantages of stability, reliability, safety, high heat exchange efficiency and the like.