| In this paper, the failure characteristics and mechanical behavior of newly-casted reinforced concrete beams and long-term service reinforced concrete beams in fire were compared and analyzed by considering the loads and heating mode. Meanwhile, the data such as pore pressure, displacement and strain development of beam specimens in fire were obtained by using the vapor pressure measuring device and a non-contact optical strain collector. The test results show that the spalling degree of newly-casted reinforced specimens depended on the water content. Due to the environmental action, the water content of long-term serviced beams was low, which made the spalling degree much smaller than that of new specimens. However, natural carbonation lead to the decrease in the strength of reinforced concrete beams in long service, and there are many fine cracks. This phenomenon makes the stressed steel bar fail more easily under the action of high temperature, which makes the concrete beam brittle and cracked. In addition, under the combined action of fire and tensile stress, the spalling time of reinforced concrete beam is advanced, while the peak value of vapor pressure decreases.