Phosphate glass in the P2O5-CaO-B2O3 (PCB) system with different B2O3 contents were prepared by melt-cooling method. The influence of B2O3 content on the structure and thermal stability was investigated. And the glass with the largest weight loss per unit area was selected for preliminary investigation of glass fiber formation process combined with high temperature viscosity and differential scanning calorimetry (DSC) analysis. The results showed that the P-O-P bond was broken down when the mass percentage of B2O3 was 3%. When it reached 12%, B3+ participated in the formation of the P-O-B bond with a greater bond strength, resulting in the highest thermal stability of the glass. At the mass percentage of 18%, [BO3] and [BO4] were present and contributed to the formation of the glass network structure. Finally, this study demonstrated the feasibility of forming phosphate glass fibers in a ternary system with PCB. It was also found that a lower drawing rate could result in defects on the fiber surface. These results provide a basis for future research on functional PCB glass fibers.