The resistance of a luminescent material to thermal quenching is essential for the application in high power LEDs. Usually, thermal luminescence quenching becomes more and more serious as the activator concentration increases. Conversely, we found here that a red phosphor Sr2P2O7:Bi2+ is one of the exceptions to this as we studied the luminescence properties at low (10-300 K) and high (300-500 K) temperatures. As Bi2+ ions are incorporated into Sr2P2O7, they exhibit the emissions at similar to 660 and similar to 698 nm at room temperature and are encoded, hereafter, as Bi(1) and Bi(2) due to the substitutions for two different crystallographic sites Sr(1) and Sr(2), respectively, in the compound. However, they will not substitute for these sites equally. At lower dopant concentration, they will occupy preferentially Sr(2) sites partially due to size match. As the concentration increases, more Bi2+ ions start to occupy the Sr(1) sites. This can be verified by the distinct changes of emission intensity ratio of Bi(2) to Bi(1). As environment temperature increases, the thermal quenching happens, but it can be suppressed by the Bi2+ concentration increase. This becomes even more pronounced in Bi2+ heavily doped sample as we decompose the broad emission band into separated Bi(1) and Bi(2) Gaussian peaks. For the sample, the Bi(1) emission at similar to 660 nm even shows antithermal-quenching particularly at higher temperatures. This phenomenon is accompanied by the blue shift of the overall emission band and almost no changes of lifetimes. A mechanism is proposed due to volume expansion of the unit cell, the increase of Bi(1) content, and temperature dependent energy transfer between Bi(2) and Bi(1). This work helps us better understand the complex luminescent behavior of Bi2+ doped materials, and it will be helpful to design in the future the heavily doped phosphor for WLEDs with even better resistance to thermal quenching.