There emerge great interests in the syntheses of metastable polyborates; however, most are involved with the high-pressure technique. A facile method to synthesize meta-stable rare earth borates at ambient pressure is eagerly required for the large-scale production and property investigation. Here we demonstrate the critical role of Bi3+ substitutions in the stabilization of metastable beta-REB3O6 (RE = Sm, Eu, Gd, Tb, Dy, Ho, Er, and Y) at ambient pressure, where the Bi3+-to-RE3+ substitutions would efficiently reduce the synthetic temperatures to 735-820 degrees C, well below the upper limit of thermodynamically stable window (840-980 degrees C). Partial solid solutions of beta-RE1-xBixB3O6 were prepared, and the ranges of the solution were also studied experimentally. The thermal behaviors of beta-RE0.8Bi0.2B3O6 were investigated by differential thermal analyses and powder X-ray diffraction, and they were divided into two categories; that is, /beta-RE0.8Bi0.2B3O6 (RE = Sm, Eu, Gd) transfers to alpha-RE0.8Bi0.2B3O6 with further increasing the temperature to 950 degrees C, while, beta-RE0.8Bi0.2B3O6 (RE = Tb, Dy, Ho, Er, and Y) decomposes into hexagonal REBO3 and B2O3. In particular, the allowed concentration of Bi3+ in beta-Gd1-xBixB3O6 was 0.10 <= x <= 0.25, and these samples show bright blue emissions under UV excitation, which suggests the high efficiency of light absorption and high potential as phosphors with further doping of other activators.