<p>Jilin Province, China, produces about 32 million tonnes of corn straw (CS) annually. Approximately 25% of this waste is openly burned, leading to both air pollution and fire hazards. This study incorporated CS as natural fibers into lightweight magnesium oxysulfide cement (MOS), which is suitable for the local cold–dry climate of Jilin Province. First, CS–MOS specimens comprising 0–15% CS were prepared to explore the effects of CS on the performance of MOS specimens. While the addition of 15% CS reduced the 28-d compressive strength by 21.8%, it significantly enhanced multiple performance indicators: the flexural–to–compressive strength ratio increased by 50.9%, the softening coefficient rose by 58.6%, the compressive strength loss after freeze–thaw cycles decreased by 69.7%, and the thermal conductivity decreased by 58.6%. Furthermore, three lightweight CS–MOS (L-CS–MOS) specimens with 15% CS and densities of 302.3, 366.7, and 533.1&#xa0;kg/m³—L3-CS–MOS, L4-CS–MOS, and L5-CS–MOS, respectively—were prepared. In accordance with the Chinese standard JGJ/T 341-2014, these specimens were classified as A03FC1, A04FC2, and A05FC3, respectively; they exhibited thermal conductivity values below 0.100, 0.125, and 0.150 W/(m·K), respectively, meeting the standard requirements. The frost resistance of L-CS–MOS notably exceeded the following specifications: after 50 freeze–thaw cycles, the compressive strength loss remained ≤17.7% (against a 25% limit for 25 cycles in the standard) and the mass loss was ≤2.5% (compared to a 5% limit).</p> Graphical Abstract <p></p>