Abstract: To investigate the influence of basalt fiber (BF) on the shrinkage behavior of lightweight aggregate concrete (LWAC), fiber-reinforced lightweight aggregate concrete (BF-LWAC) was prepared using low water absorption lightweight aggregates (LWAs) and BF. The effects of different fiber volume fractions (0.1%, 0.2%, and 0.3%) on the internal relative humidity (RH) and autogenous shrinkage behavior of BF-LWAC were systematically studied. Based on the Laplace-Kelvin equation and experimental results, a quantitative model correlating autogenous shrinkage with RH for BF-LWAC was established. The results indicate that low water absorption LWAs, after pre-wetting treatment, can effectively maintain their internal RH. Under identical net water consumption conditions, the long-term RH can be increased by 4.6%, while autogenous shrinkage is reduced by 13.6%. The moment when RH decreases to 99% was defined as the humidity critical moment. It was observed that the humidity critical moments for specimens with different BF content are similar, approximately 108 hours. After the humidity critical moment, the RH decline rates of the fiber group specimens were higher than those of the reference group, suggesting that the incorporation of BF intensifies the self- desiccation effect in LWAC. Low-field nuclear magnetic resonance method was employed to obtain the cumulative effective pore volume ratio responsible for shrinkage at 28 days. Calculation results demonstrate that the effective shrinkage stress of specimens with BF content of 0.1%, 0.2%, and 0.3% is higher than that of the reference group, yet their autogenous shrinkage is reduced by 11.7%, 16.6%, and 19.5%, respectively, compared to the reference group. This confirms that the restraint effect of BF on LWAC shrinkage significantly outweighs the deformation induced by RH changes.