The invention discloses a method for manufacturing a lamp tube for a lamp with a cover, including: taking a tube body with enclosed openings, exhausting and sealing off after filling mercury in pins, then inserting the exhaust tube of the lamp tube after sealing off in an oven with a temperature of 200-400 DEG C, and heating till amalgam and glass at the top of the exhaust tube fully melt, vibrating the tube body so as to enable the amalgam and the glass at the top of the exhaust tube compact without gaps, shifting out from the oven and naturally cooling, after cooling, flattening the exhaust tube below the amalgam, which has great benefit for improving lumen maintenance of an overall lamp and prolonging the service life of the lamp.