In order to improve the full utilization of wheat straw, to enhance the mechanical properties of gypsum board and to increase the fire resistance of wood-based gypsum board, a new preparation process of gypsum-based composites with wheat straw was put forward in this study. The thick and small wheat straw fiber were used as the reinforcing materials in gypsum-based composites, and their dimension were in the range of 0.30-0.60 and 0.18-0.30 mm, respectively. The gypsum was employed as the continuous phase in the composites. The ambient temperature curing structural adhesive was used to replace the traditional gypsum retarder in the new preparation process of gypsum-based composites. The preparation process consisted of an initial forming stage for initial strength and a curing stage, where the objectives were gypsum hydration and the strength further increased, respectively. Compared with the conventional gypsum composites with Chinese fir fiber and Chinese fir particle, the mechanical properties and fire resistance of gypsum-based composites by adding wheat straw fiber with different dimension were tested and analyzed. The results showed that the physical and mechanical properties of gypsum-based composites with small wheat straw fiber were lower than those of gypsum-based composites with Chinese fir fiber, but were better than gypsum-based composites with thick wheat straw fiber. The internal bond strength, modulus of rupture, modulus of elasticity and thickness swelling of gypsum-based composites with small wheat straw fiber were 0.33 MPa, 7.1 MPa, 2 370 MPa and 2.82%, respectively, and could meet the requirements of gypsum particleboard standard. According to the lower thermal conductivity and higher ash content of wheat straw than wood materials, the time to ignition of gypsum-based composites with wheat straw was higher than that of gypsum-based composites with Chinese fir fiber, and there was no obvious peak heat release rate in the fire resistance test. Compared with the gypsum-based with Chinese fir fiber and Chinese fir particle, the total heat release of gypsum-based composites with small wheat straw fiber was 48.18% and 35.87% lower, respectively. The main peak of CO2 production rate was 42.25% and 38.81% lower than that of Chinese fir fiber and particle gypsum composites, respectively. The main peak of CO production rate was comparable to wood-gypsum composites. The weight percent retains of small wheat straw fiber gypsum composites was slightly higher than that of Chinese fir gypsum composites, and the appearance after burning test was more complete and the degree of carbonization was lighter obviously. Due to the smooth outer surface of the wheat straw, the gypsum was mainly attached to the internal surface of the wheat straw by microscopic observation using scanning electron microscope. Therefore, the small straw fiber could increase the contact area between the internal surface and the gypsum, which indicated the inherent reason for the better physical and mechanical properties of small wheat straw fiber gypsum composites. The smooth outer surface of the wheat straw was almost covered with SiO2, which also explained the superior fire resistance of wheat straw gypsum composites. The study can be a reference for the performance improving on the gypsum-based composites with wheat straw fiber and its application in project.