Color-tunable ultralong-lifetime room temperature phosphorescence (URTP) materials have become powerful candidates for the extensive applications of optoelectronics, information encryption, anti-counterfeiting and sensing. Nevertheless, it hard to prepare URTP luminophores with both long afterglow lifetime and high phosphorescence quantum yield (PQY) under ambient conditions. Herein, we developed a convenient, rapid and gram-scale method to successfully obtain a series of URTP boron-doped carbon dots (BDs, named as BD50, BD100, BD150 and BD300, respectively), utilizing boric acid (BA, 3 g) and citric acid (CA, 50, 100, 150 and 300 mg) as precursers. BD50 shows prominent multi-colour emissive photoluminescence (PL) and phosphorescence, with extraordinarily high PL quantum yield (PLQY, 40.4%) and PQY (23.5%). Moreover, BD50 exhibits bright yellow-green afterglow, with lifetime as high as 1.17 s. What worth more to joy and encourage is that significant improvements in PQY and lifetime were achieved by introducing halide ions into BDs, due to external heavy-atom effect (EHE). BD-X (X=Cl, Br and I) possesses high PQY and long lifetime (up to 34.4% and 2.61 s). Based on the unique RTP performance of BDs and BD-X, these URTP luminophores are successfully applied to anti-counterfeiting and information encryption. This work contributes to develop smart materials with ultralong afterglow and high phosphorescence efficiency.