<p>Mullite fiber is one of the most outstanding oxide ceramic fibers for maintaining high-temperature performance while retaining flexibility. However, the rapid grain growth within mullite fibers inevitably compromises their flexibility, making the suppression of grain growth a significant challenge. Here, we develop high-performance mullite fibers with excellent flexibility at 1500&#xa0;°C by optimizing the precursor-to-polymer molar ratio via solution blow spinning. Proper ratio control reduces excessive polymer content, minimizes pore defects, and, more importantly, suppresses grain growth during high-temperature treatment, thereby enhancing thermal stability and preserving fiber flexibility. The mullite fiber sponges exhibit excellent compressive resilience, high-temperature thermal insulation, and sound absorption properties. The sponges withstand 1000 compression-recovery cycles, achieve an ultralow thermal conductivity of 0.028 W m⁻¹&#xa0;K⁻¹, and demonstrate a sound reduction coefficient of up to 0.82. This study highlights the optimization of precursor-to-polymer ratios as a promising strategy to enhance mullite fiber properties, providing valuable insights for efficiently producing high-performance ceramic fibers and potentially extending to the fabrication of other ceramic fiber materials.</p>