The invention discloses a preparation method of a rice and wheat straw active carbon ecological building material for PM2.5 adsorption and radiation prevention. The preparation method comprises the following steps: 1, uniformly mixing raw materials; 2, weighing the raw materials; 3, hotly pressing a blank; 4, post-treating the blank. According to the preparation method, rice and wheat straws are utilized for making active carbon powder, so that the pollution of the rice and wheat straws on the environment is alleviated, the utilization rate of the rice and wheat straws is increased, and a preparation process for the active carbon ecological building material is simplified; the prepared active carbon ecological building material is firm and unlikely to be damaged, has relatively high adsorption property, can effectively adsorb PM2.5 particles, and has very high radiation-proof performance; the ecological building material can be applied to numerous fields of decorative materials, building materials, artware materials, wastewater treatment materials, air purification materials and the like.