<p>Basalt fibers were added to the phosphogypsum composite cementitious material to investigate the effects of the fiber length and dosage on the flexural and compressive strengths, and thermal conductivity. The effect mechanisms of basalt fibers on the strength and thermal conductivity of basalt fiber-reinforced phosphogypsum cementitious composite were revealed. The experimental results showed that the addition of basalt fibers improved the flexural and compressive strengths, while impact on the flexural strength was more significant. The maximum flexural strength was when 1.5% basalt fibers of 12 mm were added. The maximum compressive strength was 10.74 MPa for the specimen with 1.0% basalt fibers of 9 mm. Additionally, the thermal conductivity with increasing the basalt fiber dosage increased, decreased, and then again increased. The comprehensive effect on strength and thermal conductivity indicated that the optimal dosage of basalt fibers was 1.0, 1.25, and 0.75% when the length of basalt fiber was 6, 9, and 12 mm, respectively. Moreover, the improvement of the strength was primarily attributed to the well bridging of basalt fibers and adhesion effect of hydration products. The effect mechanism of basalt fibers on thermal conductivity mainly depended on the heat transfer mode affected by the internal pore structure of basalt fiber-reinforced phosphogypsum-based cementitious composite. These results can provide the theoretical foundations for expanding the practical application of phosphogypsum in the insulated building structure.</p>