The magnesium oxide powder with low(hydration) alkalinity was produced by the reaction of brine with lime, then the composite cementitious materials as wall structure was prepared by mixing the MgO powder with straw and brine. The effect of alkaline environment on the structure and properties of the straw composite cementitious material was investigated. The results show that by controlling precipitation reaction end point with the pH less than 10.0, the magnesium oxide powder thus produced will for sure have low hydration alkalinity. Strong alkaline environment(pH greater than 12.0) has strong erosion on straw fiber, so that it has adverse effect on the mechanical properties of composite materials.Low alkaline environment(pH less than 10.0) is very adaptable for straw fiber. With the increase of use level of straw fiber, the composite material porosity increases, and the flexural strength and compressive strength decrease. The straw fiber with smaller size has great effect on flexural strength, while the straw with larger size has great effect on compressive strength of the composite materials.