A high‐temperature‐resistant flexible substrate is a critical component of high‐performance flexible electronic components. Herein, a method that does not require flowing gas and which is suitable for volume production is presented for the preparation of several‐millimeter‐long silicon nitride (Si3N4) fibers with a cross‐sectional dimension of 0.1–1.0 μm. Further, a square sheet of paper with a weight of 0.050 g, side lengths of 105 mm each, and thickness of 15 μm is fabricated using the Si3N4 fibers via a conventional laboratory paper‐making method. Subsequently, the Si3N4 paper is coated with an LaNiO3 sol using a blade‐coating machine, dried at 80 °C for 10 min, and then annealed at 650 °C for 60 min to prepare Si3N4 fiber paper/LaNiO3 composite films to evaluate the feasibility of applying the paper as a substrate for fabricating flexible functional films. The experimental results confirm that the Si3N4 fiber paper has excellent high‐temperature resistance during annealing in air, and the Si3N4 paper/LaNiO3 composite film presents excellent flexibility and conductivity. Thus, membranous flexible electronic components can be fabricated from Si3N4 fiber paper/LaNiO3 paper by a regular high‐temperature annealing process.