The present invention discloses a lamp tube of a covered bulb, including a lamp body and a core column, with a filament and an exhaust pipe on the core column, characterized in that the exhaust pipe has a bent shape. The lamp tube of the covered bulb of the present invention can reduce the working temperature of the amalgam, and increase the difference between melting temperature and working temperature of the amalgam. Besides, it can also avoid the flow of the melted amalgam when the lamp is on for a long time. It is important that the lamp tube of the covered bulb for the invention can ensure that the amalgam is at the optimum working temperature, the saturated mercury vapor pressure reaches the optimum state, whereby the consistence of photo-electricity parameter will be considerably increased. It can increase the luminous flux maintaining rate for the whole lamp to prolong the service life of the lamp, because the lamp tube of the covered bulb of the invention is often placed downward during use. The amalgam cannot flow towards the lamp tube after it melts. The structure of the exhaust pipe of the lamp will limit the flow of the amalgam. The operation is simple during assembling, easy to learn and the cost is low.