This study aimed to improve straw-based fiberboard properties without resins by adding pulping effluent as well as to investigate the difference among boards under variable hot-pressing temperatures. The characterization of fiberboards produced from wheat straw under pressing temperatures ranging from 160 to 200 °C was first described. The surface appearance, surface chemistry, thermal transitions, and mechanical performance of the boards were evaluated to investigate the effect of varying hot-pressing temperature. The results indicated that the surface color of boards became darker when the temperature was above 190 °C. Additionally, Fourier transform infrared (FT-IR) measurements showed that more low-molecular constituents and hydrogen bonds were produced under higher pressing temperatures. Furthermore, the physical and mechanical property data were analyzed statistically using one-way analysis of variance (ANOVA) and Tukey’s tests (α = 0.05). The results demonstrated that straw-based fiberboards with effluent under 190 °C exhibited superior strength and water resistance capacities, and showed great potential in commercial decorating and packaging applications.